feat: add option stdin for fmt command

[ldez]

Adds an option --stdin for the command fmt.

When using --stdin the output is always stdout.

Format Go source files

Usage:
  golangci-lint fmt [flags]

Flags:
  -c, --config PATH      Read config from file path PATH
      --no-config        Don't read config file
  -E, --enable strings   Enable specific formatter
  -d, --diff             Display diffs instead of rewriting files
      --stdin            Use standard input for piping source files