user-defined docs

[tj]

to append to what's already there, for documenting sub-commands etc

[leesei]

I added an epilog field to display available subcommands.
https://github.com/leesei/commander/commit/ddda933646870c208e2fb18615dc786d5bbade97 on branch subcmd

Then I do the subcommands parsing with my own code.
I can submit PR if you find it useful.

Possibly related to #11.
This approach do not support subcmd specific option, I've do document that in the option description.
Is there a easy way to do so?