seq: move help strings to markdown file

[m11o]

seq --help outputs follow.

$ ./target/debug/seq --help
Usage: ./target/debug/seq [OPTION]... LAST
       ./target/debug/seq [OPTION]... FIRST LAST
       ./target/debug/seq [OPTION]... FIRST INCREMENT LAST";

Arguments:
  [numbers]...

Options:
  -s, --separator <separator>    Separator character (defaults to \n)
  -t, --terminator <terminator>  Terminator character (defaults to \n)
  -w, --widths                   Equalize widths of all numbers by padding with zeros
  -f, --format <format>          use printf style floating-point FORMAT
  -h, --help                     Print help
  -V, --version                  Print version

[m11o]

-f options is implemented in #2918