Add support for labels

This merges master into @matflores fine work from #564 and also adds an example showing it in action.

As built, this only works for labels that are set up in the format ``<label for="field_id">`` and does not support the format ``<label><select /></label>``. Should we include as-is or should it be built in a way that supports the "inside" style?

This is going to close #257