given a list of tags that the user has specified, return two lists:
        matched_tags:   tags were found within the current play and match those given
                        by the user
        unmatched_tags: tags that were found within the current play but do not match
                        any provided by the user

def compare_tags(self, tags):
        ''' given a list of tags that the user has specified, return two lists:
        matched_tags:   tags were found within the current play and match those given
                        by the user
        unmatched_tags: tags that were found within the current play but do not match
                        any provided by the user '''

        # gather all the tags in all the tasks into one list
        all_tags = []
        for task in self._tasks:
            all_tags.extend(task.tags)

        # compare the lists of tags using sets and return the matched and unmatched
        all_tags_set = set(all_tags)
        tags_set = set(tags)
        matched_tags = all_tags_set & tags_set
        unmatched_tags = all_tags_set - tags_set

        return matched_tags, unmatched_tags