Underscore support for pattern-lab/patternlab-node#623

Author: bmuenzenmeyer

lib/engine_underscore.js

@@ -87,7 +87,7 @@ _.mixin({
 var engine_underscore = {
   engine: _,
   engineName: 'underscore',
-  engineFileExtension: '.html',
+  engineFileExtension: ['.html', '.underscore'],
 
   // partial expansion is only necessary for Mustache templates that have
   // style modifiers or pattern parameters (I think)