Add Furbooru support

[BytewaveMLP]

The Derpibooru team have recently branched out to the furry community with Furbooru, a furry image booru using Philomena. Since the API is currently 1:1 compatible, it should be easy enough to adapt node-derpi to accept an arbitrary API endpoint URL, and just reuse the same code.

Considerations to be made:

• Should this be a separate instance of the bot?
• Should existing commands be adapted to allow for Furbooru searching?
• Should new commands be added entirely for Furbooru?
• Should existing Derpi commands be renamed for consistency?