switch to less heavy alternative to clap

[extrawurst]

something like https://github.com/RazrFalcon/pico-args or https://github.com/google/argh

just by removing clap the release binary size shrank by 500k

[studoot]

gumdrop is another possible option - this blog post suggests a 500kB size reduction as well.

[extrawurst]

let's keep this help output as similar as possible:

gitui 0.8.1
Stephan Dilly <dilly.stephan@gmail.com>
blazing fast terminal-ui for git

USAGE:
    gitui [FLAGS] [OPTIONS]

FLAGS:
    -h, --help       Prints help information
    -l, --logging    Stores logging output into a cache directory
    -V, --version    Prints version information

OPTIONS:
    -d, --directory <directory>    Set the working directory

and release/stripped bin: 1,8mb

[extrawurst]

ok I tried pico-argh and it reduces the raw binary size by 400kb (200kb after stripping), but does not reduce the dependencies at all (other than textwrap, which we soon need anyway due to #181). also the compile time only gets reduced by 7s. on the other hand convenience gets reduced significantly so I for now decide that the gain is not worth the pain