change tag pattern regexp to allow unquoted attribute values

This pull request is only for the changes to `components/prism-markup.js`; I'm not sure how to tell github to ignore the other changes.  The current pattern uses `\w+` but the [html5 grammar](http://www.w3.org/html/wg/drafts/html/master/syntax.html#attribute-value-(unquoted%29-state) allows just about any character, `[^\s'">=]+`