Add support for linkifying unicode tags

Per ["Auto-linkification of Tags.TagName fails when the tag name contains nonstandard characters"](http://meta.stackexchange.com/questions/226703/auto-linkification-of-tags-tagname-fails-when-the-tag-name-contains-nonstandard). Adding XRegExp just for this is a *tad* overkill, but it has some nifty features I want to use elsewhere to clean up some of the JavaScript mess, so bear with me on that one.