HTMLHelper mechanism to add non-data attributes to labels

This is essentially the same as Coldbox-545 but for the label itself rather than wrapper tags.

For instance, if I want to slap a title or an id attribute on a label, I don't think I can do that at the moment with HTMLHelper. I propose adding labelAttrs to the HTMLHelper form element tags that work the same way as wrapperAttrs – we just flatten an incoming struct and draw them as attributes when opening the label tag.