Better locale guessing

The werkzeug best_match method gives unsatisfying results when requested and available languages regions differ.

>>> from werkzeug.datastructures import LanguageAccept
>>> accept = LanguageAccept([('fr-FR', 1), ('en-US', 0.7), ('en', 0.3)])
>>> available = ['gl', 'de', 'fr', 'es', 'nb_NO', 'en']
>>> accept.best_match(available)
'en' # <-- This should be 'fr'

We should add a layer to fix this.