Filter out control characters (U+0000 to U+001F) from outgoing
messages, as they are illegal in XML 1.0 (except for \t, \n, and \r).
This prevents XMPP servers from closing the connection when such
characters are accidentally or intentionally included in a message.
Fixes: https://github.com/profanity-im/profanity/issues/1437
189050f3f4