Collapse long messages into gist

[emmiegit]

Add a command which allows specifying a range of messages by the same user. The concatenated content of all the messages to be uploaded to a gist, and then the original messages deleted. The goal is to make it easy for moderators to "move" overly long code snippets or other information to an external link.

Requires the "manage messages" permission if being performed on a user that is not yourself.
Requires #108