Add descriptions to custom search engines

Configuration:

    # With description...
    w: http://www.wikipedia.org/w/index.php?title=Special:Search&search=%s wikipedia
    
    # Without description...
    yt: http://www.youtube.com/results?search_query=%s

Any tokens after the URL are taken to be the description...

![snapshot](https://cloud.githubusercontent.com/assets/2641335/5570729/359a1716-8f7e-11e4-8f4b-7b409b1e5ba9.png)

Absent a description, it just looks like it did before...

![snapshot](https://cloud.githubusercontent.com/assets/2641335/5570737/63748540-8f7e-11e4-9662-4ceccc97119d.png)

Issues:
- It might be nice to have styling similar to chrome.  However, the styling should also be consistent with that of the rest of our suggestions. Ho, hum.
- We activate the search engine on `w`.  Chrome activates it on `w<Space>`.  Should we change to chrome's approach?