Added ability to specify a headed browser.

Added the ability to specify a headed browser (other than the default: chrome)
See how mccadmin would use this change [here](https://github.com/mdsol/mccadmin/compare/feature/kender_specify_headed_browser?expand=1): 
Other front-end apps could follow a similar approach.
b4ccb982f9149f3871a798a2a0f1adee26aef78d