update directive priority and replace transcluded element content if it exists

If an element included both an `i18ng` directive and another directive that used a transcluded template, the transcluded template was getting overwritten by the translation. Apparently, transcluded templates are rendered before directives are executed, so even with a higher priority, i18ng was still overwriting the html of the transclusion. The fix here adds the priority AND THEN looks for a `[ng-transclude]` element. It will apply the translation to the transclude element, or to the base element if one is not found.