feat(config): log if no config file is specified

Now that Karma tries to be a bit more clever and tries `karma.conf.js` or `karma.conf.coffee`, it's helpful to know that no config file was loaded.

lib/config.js

@@ -323,6 +323,7 @@ var parseConfig = function(configFilePath, cliOptions) {
       return process.exit(1);
     }
   } else {
+    log.debug('No config file specified.');
     // if no config file path is passed, we define a dummy config module.
     configModule = function() {};
   }