Add support for sending custom Firefox Profiles to Sauce Labs

Suggestion for adding support for custom firefox profiles. I am not sure the best way to add this feature, so please let me know.  

The diff got a little messy because I had to move the driver code around.  Let me know if you would like me to do this in a different way. 

This Pull Request addresss https://github.com/karma-runner/karma-sauce-launcher/issues/59

Thanks