Add positional args for directory, branch, and repository.

[matro]

This implements [<options>] [<directory> [<branch> [<repository]]] but requires all <options> come before the positional args.

Fixes part of X1011/git-directory-deploy#14.

[matro]

This feel a little hacky to me, but it seems to work in my testing.

[X1011]

as with #19, i'm going to wait until we have tests for order of preference to merge this, cause i think the env variables will override these arguments as it is currently implemented.

[matro]

Got some tests committed here, though I'm still working on the positional args themselves.

[matro]

I've merged master into this, and added a couple more commits. I think this is working now. All tests pass (including one specifically for the positional args), and there're docs for this.