lightbox-plugin: Basic Lightbox support

Lightbox works for image tags with a class of the form ``lb-<gallery>``
where ``<gallery>`` is the Lightbox gallery the image will be placed in.
    The Lightbox files still need to be added manually.
Changed ``gallery`` to ``set``.
Wrote a useful readme file.
Wishlist update
Merge branch 'ligthbox-plugin'