Adding a discord adapter

[bensontrinh]

Task:

Getting a discord adapter implemented.

I'm using the library from discljord to implement a lot of this.

Todo before merging:

• [x] Add some test coverage
• [x] Cleanup code
• [x] Implement reaction support

[devth]

Note: adapters now pass the raw event in the chat-source so cmd handlers can optionally use platform-specific stuff, so we should do that for discord as well, e.g. https://github.com/yetibot/core/pull/224/files#diff-1a12310cc4d4bb44655929798120810eab79f8ad290cb8e65a3605d8fc3a34f0