Initial implementation

This module serves two purposes

 - I'm going to make 100% compat with play-doh open source standards. Thus setting an example for other modules
 - it fixes a painpoint writing tests for CLIs that work with files, git & npm.

The tests run and have 100% branch coverage.

cc @sh1mmer @jcorbin would love an implementation / readme review

cc @twolfson `exec-callback` is being shelfed for now, would love to move your "open source best practices" feedback to this PR instead of the `exec-callback` PR.