Support custom frameworks

SauceLabs allows for [custom frameworks](https://saucelabs.com/docs/rest#jsunit), but that has been missing from `grunt-saucelabs` so far. This adds support for it.

The test is based off of mocha, as SauceLabs support for mocha is pretty bad as you can see from [your own tests](https://saucelabs.com/tests/6c5da543483d43ffae2a6b20443fc7bb).