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.