Make taggle compatible with xhtml

[FranklinWhale]

This PR allows Taggle to be used with a page served as application/xhtml+xml by:

1. Replacing the × HTML entity with the × character directly

2. Changing the tagFormatter return validation logic to check localName, which works in both application/xhtml+xml and text/html, and for compatibility with IE 8, tagName as fallback.

Fixes #129

[coveralls]

Coverage remained the same at 89.957% when pulling f66287d on FranklinWhale:xhtml into ebfa55c on okcoker:master.

[okcoker]

Can we make sure the localName and tagName get lowercased so we can compare apples to apples. I've never actually used localName before but MDN says it was once uppercase.

[FranklinWhale]

tagName is always uppercase in HTML. It is likely lowercase in XHTML, but if the element is namespace prefixed (e.g. <x:li>), the prefixed tag name (x:li) is returned. In contrast, localName never returns the namespace prefix.

localName should be in lowercase for both HTML and XHTML if the browser conforms to the HTML or DOM specification. IE >=9 and the latest versions of Edge, Firefox and Chrome all meet the requirement. Since the check falls back to tagName, browsers that do not localName or has incorrect implementation will work just fine.

[FranklinWhale]

Thanks! This fixed #129