Update rustfmt command line interface

[topecongiro]

This PR updates the rustfmt command-line interface in the following ways. Sorry in advance about the number of changes!

• Remove --skip-children.
• Remove --help=[command] and introduce the following alternatives:
  • --help=config can now be invoked via --config help
  • --help=file-lines is now shown with --help by default
• Change the behavior of --print-config
  • Remove the output file argument: now always print to stdout
  • --print-config minimal prints the set of current configuration that differs from the default, without formatting files.
  • --print-config default prints the set of all configuration options with the default value
  • --print-config current prints the set of the current configuration, without formatting files
• Remove --unstable-features.
• Remove --color.
• --help now prints longer help messages
  • This change is a side-effect of replacing getopts with structopt. Please see the following comment on how it changed.
• Change the behavior of Config::used_options.
  • Return a set of configuration options that were explicitly set to values that are different from the default value

[topecongiro]

rustfmt -h:

rustfmt 2.0.0-rc.1-nightly (265697c2 2020-01-08)
Format Rust code

USAGE:
    rustfmt [FLAGS] [OPTIONS] [--] [files]...

FLAGS:
    -c, --check                   Run in 'check' mode
        --error_on_unformatted    Error if unable to get comments or string literals within max_width, or they are left
                                  with trailing whitespaces (unstable)
    -l, --files-with-diff         Prints the names of files with diff
    -h, --help                    Prints help information
    -q, --quiet                   Print no output
    -r, --recursive               Recursively format submodules
    -V, --version                 Prints version information
    -v, --verbose                 Print verbose output

OPTIONS:
        --edition <2015|2018>                       Rust compiler edition
        --config-path <config-path>                 A path to the configuration file
        --print-config <default|current|minimal>    Print configuration options
        --emit <emit>                               Specify the format of rustfmt's output
        --file-lines <file-lines>                   Limit formatting to specified ranges [default: null]
        --config <inline-config>...                 Set options from command line

ARGS:
    <files>...    

rustfmt --help:

rustfmt 2.0.0-rc.1-nightly (265697c2 2020-01-08)
Format Rust code

USAGE:
    rustfmt [FLAGS] [OPTIONS] [--] [files]...

FLAGS:
    -c, --check                   
            Run in 'check' mode.
            
            Exits with 0 if input is formatted correctly. Exits with 1 and prints a diff if formatting is required.
        --error_on_unformatted    
            Error if unable to get comments or string literals within max_width, or they are left with trailing
            whitespaces (unstable)
    -l, --files-with-diff         
            Prints the names of files with diff

    -h, --help                    
            Prints help information

    -q, --quiet                   
            Print no output

    -r, --recursive               
            Recursively format submodules.
            
            Format all encountered modules recursively regardless of whether the modules are defined inline or in
            another file.
    -V, --version                 
            Prints version information

    -v, --verbose                 
            Print verbose output


OPTIONS:
        --edition <2015|2018>                       
            Rust compiler edition
            
            Specify which edition of the compiler to use when formatting code.
        --config-path <config-path>                 
            A path to the configuration file

        --print-config <default|current|minimal>    
            Print configuration options.
            
            `default` will print the default configuration options. `current` will print the current configuration
            options. `minimal` will print the minimal subset of the current configuration options.
        --emit <emit>                               
            Specify the format of rustfmt's output

        --file-lines <file-lines>                   
            Limit formatting to specified ranges.
            
            If you want to restrict reformatting to specific sets of lines, you can use the `--file-lines` option. Its
            argument is a JSON array of objects with `file` and `range` properties, where `file` is a file name, and
            `range` is an array representing a range of lines like `[7,13]`. Ranges are 1-based and inclusive of both
            end points. Specifying an empty array will result in no files being formatted. For example,
            
            ``` rustfmt --file-lines '[ {{\"file\":\"src/lib.rs\",\"range\":[7,13]}},
            {{\"file\":\"src/lib.rs\",\"range\":[21,29]}}, {{\"file\":\"src/foo.rs\",\"range\":[10,11]}},
            {{\"file\":\"src/foo.rs\",\"range\":[15,15]}}]' ```
            
            would format lines `7-13` and `21-29` of `src/lib.rs`, and lines `10-11`, and `15` of `src/foo.rs`. No other
            files would be formatted, even if they are included as out of line modules from `src/lib.rs`. [default:
            null]
        --config <inline-config>...                 
            Set options from command line.
            
            Set configuration options via command line by specifying a list of key-value pairs separated by commas
            (e.g., rustfmt --config=max_width=100,merge_imports=true). These settings precedes any other settings
            specified in configuration files.

ARGS:
    <files>...