Add/desk - adding integration for Desk.com

Adding index.js and associated tests for Desk.com
This is pretty obscure method that involves knowing a bunch about how our ingestion works, but in this case, we don't need to  actually implement it. The reason is its prototype already checks that the integration is enabled for this message [here](https://github.com/segmentio/integration/blob/master/lib/proto.js#L104), and we want Desk to process the web, server, and mobile channels so we don't need the `'server' == msg.channel()` check.