Markdown support

[DanWin]

Markdown is a great option for user-friendly styling of messages, which is commonly used. This could be manually implemented with regex filters, but would be a nice feature to be part of the core.
This feature should be possible to turn on/off on admin level, as well as on user level, to allow users switching between plain-text and markdown input.

[rexzooly]

I know it was Jan when you posted this but I would like to push this to your mind :)