Query colorless cards

[WPaul15]

Right now, the API represents colorless cards by not including any color information. That's fine if the client is simply pulling everything, but in doing this, it prevents clients from filtering for colorless cards.

I propose that "Colorless"/"C" be included as colors in the results and support be added for these to be included in the colors and colorIdentity query params. It looks like MTGJSON supports colorless cards already.