Bulk fetch and cache prices for MTG cards/decks

Card data version 2.11.3 from mtgjson.com, slightly modified (reformatted). Pretty-printed using github.com/broady/json_prettyprint

Notes:

• No proper error handling in the UI. Some cards may not show up.
• Need to add retries for the HTTP calls to pricing and Gatherer.

$ go get github.com/broady/mtgprice
$ mkdir ~/.mtg && cd ~/.mtg
$ mtgprice