docs: Replace Crate.io with PyPI
plcont: Use self.__len__() instead of len(self) to allow for easier mocking
plcont: Make remove_playlist() docstring match behavior better
plcont: More docs on del usage
plcont: Add __setitem__ support
plcont: Adjust __delitem__ tests to not test too much
plcont: Add insert()
plcont: Subclass collections.MutableSequence
plcont: Mention insert() and append() in docs