Allow HTML inside attributes. Fixes #1322

This includes a breaking change - since attributes were parsed using `element` converter first, it was possible to use characters that aren't allowed inside unquoted attributes, e.g. `<div data-attr=<p></p>></div>` would be parsed correctly. This is no longer the case as it was probably a bug too. @ractivejs/team-ractive let me know if you think we should keep on supporting that.