Enhacement: Add remove_by_slug() method to TaggableManager

That way it would also be possible to remove tags by providing their unique slugs. Have in mind that people might want to use URLs like .../some-object/tags/my-precious-tag/remove/ to remove a tag.

Adding this method seems easy, but I am reluctant to contribute a PR because I am not sure if I can write correct tests for this code addition. If some guidance and supervision is provided I am willing to try.