Add functionality for widget instances to have unique templates

I needed a way to use unique templates for each Taxonomy Drill-Down widget. Inspired by WordPress' [`get_template_part`](http://codex.wordpress.org/Function_Reference/get_template_part) I added the ability to do something similar in the widget instance config. It is now possible to add a 'suffix' parameter to each widget which will make the template renderer look for the functional equivalent of `"qmt-templates/{$mode}-{$suffix}.html"` before the default template.