Fast Pixiv source searches and pixiv: metatag

This addresses #726.  For single images, or images belonging to a single manga post, it adds a pixiv: metatag to quickly find posts by Pixiv ID.  The index makes this just as fast as any other search on a numeric property.

For artist searches or other general searches, it adds the ability to search sources in the format source:pixiv/[query], which is equivalent source:*pixiv.net/img*/[query].  This makes the query left-anchored, which allows it to be indexed and sped up dramatically.  It also attempts to catch source queries formatted in the old style and map them to the new style, so this should be a completely turn-key solution.