Specify multiple configuration and merge them for nightwatch run

[kpiwko]

I was thinking of adding capability to the runner to merge configurations from nightwatch.json.

It could work as a merge of two (or more) configuration envs present in nightwatch.json, creating new nightwatch-run.js file that would be passed to nightwatch by using --config ./nightwatch-run.js. Also, it would allow to specify selenium configuration in env, so for instance override webdriver.chrome.driver in chrome-ci env.

Then, in your project using grunt-nightwatchjs, you could specify a task and call it like

grunt seleniumTest:serverEnv+browserEnv

Let me know if I can proceed with implementing this feature request.