Use templates to generate enclosures

This has two benefits:
  - make it possible to handle multiple enclosures;
  - handle _only_ the anchors with the 'data-expanded-url' attribute,
    before the change every anchor with the 'twitter-timeline-link'
    attribute was handled.

The change also makes the DOM navigation a little lighter because now
only $tweet-text is searched for the 'data-expanded-url' attribute.