unexpand: move help strings to markdown file

[piotrkwiecinski]

#4368

unexpand --help outputs the following:

target/debug/unexpand --help
Convert blanks in each `FILE` to tabs, writing to standard output.
With no `FILE`, or when `FILE` is `-`, read standard input.

Usage: target/debug/unexpand [OPTION]... [FILE]...

Options:
  -a, --all             convert all blanks, instead of just initial blanks
      --first-only      convert only leading sequences of blanks (overrides -a)
  -t, --tabs <N, LIST>  use comma separated LIST of tab positions or have tabs N characters
                        apart instead of 8 (enables -a)
  -U, --no-utf8         interpret input file as 8-bit ASCII rather than UTF-8
  -h, --help            Print help
  -V, --version         Print version

[cakebaker]

Thanks!