feat(task): process templates in the config

Templates in the preprocessors config are now
properly processed.

tasks/grunt-karma.js

@@ -60,6 +60,18 @@ module.exports = function(grunt) {
       data.files = _.flatten(data.files);
     }
 
+    // Allow the use of templates in preprocessors
+    if (_.isPlainObject(data.preprocessors)) {
+      var preprocessors = {};
+      Object.keys(data.preprocessors).forEach(function (key) {
+        var value = data.preprocessors[key];
+        key = path.resolve(key);
+        key = grunt.template.process(key);
+        preprocessors[key] = value;
+      });
+      data.preprocessors = preprocessors;
+    }
+
     //support `karma run`, useful for grunt watch
     if (this.flags.run){
       runner.run(data, finished.bind(done));