1) It's about time I separated VALID_FOO_URL_REGEX into a list of
   possible regular expressions.

2) After #1 is done, the owns_url? method can be factored out.

3) Merge the Mentalzero get_video_filename with the algorithm in the
   Xvideos class. Only the last lines differ. Maybe we could have a
   collection of filename-getting algorithms.