split: move help strings to markdown file

[m11o]

issue: #4368

split --help outputs follow.

$ ./target/debug/split --help
Create output files containing consecutive or interleaved sections of input

Usage: ./target/debug/split [OPTION]... [INPUT [PREFIX]]

Arguments:
  [input]   [default: -]
  [prefix]  [default: x]

Options:
  -b, --bytes <SIZE>
          put SIZE bytes per output file
  -C, --line-bytes <SIZE>
          put at most SIZE bytes of lines per output file [default: 2]
  -l, --lines <NUMBER>
          put NUMBER lines/records per output file [default: 1000]
  -n, --number <CHUNKS>
          generate CHUNKS output files; see explanation below
      --additional-suffix <SUFFIX>
          additional SUFFIX to append to output file names [default: ]
      --filter <COMMAND>
          write to shell COMMAND; file name is $FILE (Currently not implemented for Windows)
  -e, --elide-empty-files
          do not generate empty output files with '-n'
  -d, --numeric-suffixes [<numeric-suffixes>]
          use numeric suffixes instead of alphabetic
  -a, --suffix-length <N>
          use suffixes of fixed length N. 0 implies dynamic length. [default: 0]
  -x, --hex-suffixes [<hex-suffixes>]
          use hex suffixes instead of alphabetic
      --verbose
          print a diagnostic just before each output file is opened
  -h, --help
          Print help
  -V, --version
          Print version

Output fixed-size pieces of INPUT to PREFIXaa, PREFIXab, ...; default size is 1000, and default PREFIX is
'x'. With no INPUT, or when INPUT is -, read standard input.

[cakebaker]

Thanks :)