HTML: Add dash to the allowed characters in tag names

In the Angular universe we use custom elements all over the place, but rouge currently thinks they're errors.

Dashes are allowed in custom elements in HTML5 per this article
http://www.html5rocks.com/en/tutorials/webcomponents/customelements/

I tried to look for something more 'spec-like' than that article but couldn't quite. But no modern browsers have problems with dashes in element names.