Add --only-if-changed option.

I added an option, `-o`, or `--only-if-changed`, to only run the playbook if the repository has changed. In my use case, there's no sense in running the playbook if the repository hasn't changed, as it's idempotent, so it won't do anything.

Hopefully this can be merged, as it would be *extremely* useful to me, and I'm sure to other people as well.