New html filter

So, I have decided to update HTML filter in ABXD. It's way better IMO than previous filter. I don't think it has security problems, the BBCode support is better (to begin with, `[url=something]<i>italic</i>[/url]` works), the code is parsed just like HTML5 specification says (provided you won't use BBCode).

Please note it's incompatible with old BBCode plugins. The new BBCode plugins should use DOM syntax, instead of plain text.