pc should have option to ignore errors from files not known to git

[masukomi]

Sometimes you don't care that the file isn't known to git. This is frequently the case with a plugin that may not be checking.

We should add a new option that indicates that pc should not return an exit status indicating failure if the only problem is that the file isn't known to git.

Instead it should return the same thing as a request for a file known to git with no comments in it.

maybe -q && --quiet ? I'm not sure if "quiet" is a good thing because its a word whose meaning might expand over time. -g & --gitless ? dunno, that seems worse. I'm open to suggestions.