Be smarter about keyword searches

[njx]

Create a "content" type of match that searches for the query in title or description, and add "content=querystr", "title=querystr", and "description=querystr" as possibilities in the dropdown, so that they'll always match the currently-typed string and will be available if no other matches work.

Calling this "usability" since right now in order to do an ordinary keyword search you have to know to type "title=" or "description=".