Showcase apps that are more well-known. Resolves #139

Added a new category in projects.json: "showcased".
Added DoubleClick, Whitetruffle, and Plunker to category.
Showcased apps are displayed at the top of the unsorted list.

I chose DoubleClick, Whitetruffle, and Plunker to be placed at the top of the unsorted category primarily because those were the only not-entirely-obscure apps I found on the list. Let me know what you think about that selection.