Maintain existing values when manipulating form elements

As things are, if you have an address form with existing values (for example, in situations where you're editing values that had been pre-populated elsewhere), those values will not necessarily be maintained when applying the plugin to the form.

This is particularly an issue when running any of the following under some circumstances / configurations:
`$.fn.addressfield.convertToSelect()`
`$.fn.addressfield.convertToText()`
`$.fn.addressfield.updateOptions()`
`$.fn.addressfield.orderFields()`

We should explicitly add support for this common scenario.  We should not maintain the value when a field is removed/hidden, however.