Allow render functions in options to return DOM.

This PR allows the functions in the render options "option", "item", "option_create", "optgroup_header", "optgroup" to return DOM nodes instead of plain strings.

This should come in handy, in contexts where we do not have strings but DOM, like in ember.js.

See:
https://github.com/miguelcobain/ember-selectize/blob/master/src/ember.selectize.js#L476
which caused
https://github.com/emberjs/ember.js/issues/5534

I have no idea what I'm going to break with these changes :)