Implement text-based interface for the crawler

[horazont]

The invitation flow can be cumbersome in some clients, because very few people will have the crawler in their roster.

In order to improve the usability, the crawler should accept text messages containing XMPP URIs or other forms of JIDs and add them to the queue.

In addition, it might do a simple check whether the JID would even be elegible:

• Is the JID reachable?
• Does it look like a MUC?
• Does it have the correct flags?

This extra functionality should respect the blocklist and should be rate-limited.