metatag_context support fieldsets from metatag sub modules

Currently the metatag_context module, when displaying the metatag form on the context options form attempts to flatten known metatag fieldsets due to an issue with context. The problem is that the current code, is written with hardcoded fieldset values that are only known from the core metatag suite of modules.

<?php
   // TODO: Perhaps it can be done in a better way with #tree and #parents?
   foreach (array('advanced', 'dublin-core', 'open-graph', 'twitter-cards') as $fieldset) {
     if (isset($form['metatags'][LANGUAGE_NONE][$fieldset])) {
?>

This means that any metatag sub modules that define a new metatag groups cannot have their values set via features (due to their fieldsets not being flattened).