Add ‘cache’ option to gitrm task

[rafegoldberg]

use case:

i have a blank JSON config file for user credentials, and a task that lets them update each entry on postinstall. once they've added passwords to this file I don't want them to accidentally push it. so it'd be useful to rm --cache it out of gits working tree, without deleting it from the actual filesystem.

but possibly I am just misunderstanding how to use this task?