add default executable (subcommand) option

[Qix-]

This PR introduces an option for the .command() call that allows git-style subcommands to default to a specific command in the event one isn't provided on the command line.

The use case that brought about this PR was a build system I'm developing needing to run the subcommand build automatically when nothing was specified on the command line. I'm sure there is a hack that could have done this with .action() but alas I wanted it to work with the executable functionality (i.e. having another script to do run the command).

I'm sure I'm not the only one who can find a use case for this.

Test is included (based off of the pm-subcommand test) and shouldn't interfere with the existing test.

Also added some documentation (including documentation for the previously undocumented opts.noHelp option).

I tried to keep the implementation as minimal as possible. Let me know if I need to change anything.

[noamokman]

👍

[SomeKittens]

LGTM