Strip HTML tags

A start on #35.
Better error handling in html stripping

Abort stripping if tokenizing errs.
Merge pull request #48 from arlolra/html

Strip HTML tags in messages.
Show XEP-0203 timestamp if given.

If a message was delayed and includes an XEP-0203 timestamp of the send
time, show that rather than the receive time.

Thanks to Peter Stuge.
Add chat, xa, dnd, online and away commands.

Thanks to Peter Stuge.