Support --reporter and --reporter-output when running tests.

When running any Grunt task that runs tests with Mocha, the two
following command line options are now supported:
- --reporter: allows to specify a reporter, default is 'spec'.
- --reporter-output: allows to specify a file where Mocha's output will
  be stored.

These two command line options make it easier to retrieve and parse test
results when tests are run on a continuous integration platform like
Jenkins.

Fixes #4038.

Gruntfile.js

@@ -171,8 +171,9 @@ var _              = require('lodash'),
             mochacli: {
                 options: {
                     ui: 'bdd',
-                    reporter: 'spec',
-                    timeout: '15000'
+                    reporter: grunt.option('reporter') || 'spec',
+                    timeout: '15000',
+                    save: grunt.option('reporter-output')
                 },
 
                 // #### All Unit tests