flatten files option as typical in grunt plugins

Since grunt-karma doesn't allow grunt to handle the source files using the standard Grunt way, you loose the automatic array flattening.  This is useful when referencing other vars in your files array (and those vars are themselves arrays).  This has happened to use in cgross/generator-cg-angular#23.  

 I think changing grunt-karma to be a good acting grunt plugin by using the standard fileSrc would be best (ala #60).  But that will require breaking changes to how users specify their options.  This change does not go that far and thus can be applied with no compatibility problems.