Allow `ember test --filter` to perform mocha `grep`.

Enabled by https://github.com/ember-cli/ember-cli/pull/2820.  This will start functioning properly in ember-cli 0.1.5 (where that PR will land in a release), but in the meantime is benign (the function added is unused).

Fixes https://github.com/switchfly/ember-cli-mocha/issues/17.

References:

http://mochajs.org/#grep-option
http://mochajs.org/#grep-query