Remove elements unused attributes in readability result

Remove elements unused attributes in readability result. For example, ` data-is` and `riot-tag` attributes in the `<a href="http://starwars.wikia.com/wiki/Celebration_Orlando" data-is="trackable" riot-tag="trackable">Celebration Orlando</a>` element.

[1]: https://github.com/mozilla/readability/blob/master/test/test-pages/wikia/expected.html#L3
