Allow <irony> tags

Hi, I created a small patch to allow for <irony> tags in discussion. As should be expected, on the first glance the text looks exactly the same as it would without the tags (since irony is not always easy to detect, and shouldn't be), but if you move your mouse pointer over the tagged text, the mouse pointer will change as to indicate a link. Clicking the tagged text leads to http://en.wikipedia.org/wiki/Irony

I thought this might be a nice gimmick to spice discussions. The execution can easily be changed, e.g. to automatically add a smiley to the tagged text to make it a bit more noticeable. 
As an additional gimmick I could implement them as "hidden" tags, e.g. not showing them in the list of approved tags but still allowing them, like an Easter-egg.

Another change I'd be happy to see is to allow <strike> tags. I didn't add them to the allowed tags with this commit, but would be happy to see that as a future change since I consider them rhetorically valuable.