Rename paths_to_mutate and add only_mutate config

[Otto-AA]

Fixes #414

Adds a only_mutate config that is a glob pattern similar to do_not_mutate:

    def test_respects_only_mutate_with_do_not_mutate(self):
        config = self._get_config(
            only_mutate=["src/api/*"],
            do_not_mutate=["src/api/models/*"],
        )
        # matched by only_mutate
        assert config.should_mutate("src/api/endpoints/user.py") is True
        # matched by only_mutate but excluded by do_not_mutate
        assert config.should_mutate("src/api/models/user.py") is False
        # not matched by only_mutate
        assert config.should_mutate("src/services/user.py") is False