Allow for specifying whether message is DM in ReceiveMessageEvent

[deuill]

Currently, the standard joe.ReceiveMessageEvent sent by all documented adapters does not specify whether the message is "private" (a one-on-one chat in Slack, a /msg in IRC, etc).

This can be useful in having handlers process commands differently (e.g. allow admin-level commands in direct chats) depending on the communication mode, without having to either parse the Channel or Data fields (which assumes the handler knows which adapter is being used) or having to use custom events, which assume adapter support.