Minimal label/value support

Here's a possible solution to the label/value request that comes up frequently (eg: https://github.com/aehlke/tag-it/issues/266, https://github.com/aehlke/tag-it/issues/160, https://github.com/aehlke/tag-it/pull/80).

This works well with a few caveats that could be dealt with in future:
*You can only provide this data to autocomplete.source, not availableTags
*You cannot currently pre-fill label-tag_id pairs
*Label-tag_id pairs only work with autocomplete
*You cannot use label-tag_id pairs with a single input field
*Does not properly prevent duplicates

I think this is a good start to the solution, the one proposed in https://github.com/aehlke/tag-it/pull/80 is from old code so may not be ideal for merging.