Add localization for DOM elements

[porst17]

This allows to localize elements that are not currently attached to the DOM tree,
such as elements just created via document.createElement() or elements that are
managed by a third party library.

The use case is something like this:

const div = document.createElement('div');
div.setAttribute('data-i18n','the.key');
localize(div);

// later
parent.appendChild(div);

I have to admit that my changes are entirely untested. I just edited and commited directly on github. Hope it works anyway.

[porst17]

Maybe this should be extended to support all kinds of iterable objects like NodeList etc.