Strip XHTML/XML formatting in all irc messages.

Not sure if I did it right, as it currently will strip any html-like things that might be there intentionally (but in unescaped form? is it even possible?).

E.g. !repos gives this to IRC channel:

	[17:05:46]<err> <!-- look here to see what is supported : http://xmpp.org/extensions/xep-0071.html -->
	[17:05:46]<err> <html xmlns="http://jabber.org/protocol/xhtml-im">
	[17:05:46]<err> <body>
	[17:05:46]<err> <div style="font-family: monospace">
	[17:05:46]<err> <p style='margin-top: 0; margin-bottom: 0;'>Repos (P = Private repo, * = installed):</p>
	[17:05:46]<err> <p style='margin-top: 0; margin-bottom: 0; white-space:pre;'>
	[17:05:46]<err> [<span style='font-weight:bold;'>  </span>] err-calcbot      a smart calculator, unit converter and math solver<br/>
	[17:05:46]<err> [<span style='font-weight:bold;'>  </span>] err-codebot      can make the bot execute code snippet in C, CPP and Python<br/>
	...

Which is kinda awful for readability.

With fix, text is cleaned of markup - only valid one, and escaped non-markup entities should be unescaped, iiuc:

	[17:07:34]<err> Repos (P = Private repo, * = installed):
	[17:07:34]<err> [  ] err-calcbot      a smart calculator, unit converter and math solver
	[17:07:34]<err> [  ] err-codebot      can make the bot execute code snippet in C, CPP and Python
	...

Can `Message.getBody()` return a valid `<tag>stuff</tag>` markup in unescaped (e.g. `&lt;tag&gt;stuff&lt;/tag&gt;`, or its xml equivalent) form from some plugin?

If so, might be not a perfect solution, but cheap easy fixes can be either to escape everything not marked as xhtml (if that's a thing) or make it configurable, shifting a bit of a burden onto user.