Consolidating element attribute copying to a method.

Both the `convertToSelect` and `convertToText` methods make assumptions about the form element attributes that should be maintained when converting element types.

They're sensible defaults, but we should allow others to add their own (or get rid of the defaults, if need be).

A configuration layer is beyond a 0.2.x release, so maybe we should just abstract it into an override-able method, e.g. `addressfield.copyAttrsTo()`.