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.
I would say that instead of adding this elsif here, you can do someithing like:
```Ruby
extra_env = ENV['HEADED_BROWSER'] ? HEADED_BROWSER=#{ENV['HEADED_BROWSER']} : ''
if defined?(ParallelTests)
'#{extra_env} bundle exec rake parallel:features'
else
 '#{extra_env} bundle exec cucumber'
end
```
So this way you use this foe both parallel and not parallel