Enable passing a single test to nightwatch

You can now pass single testfiles to nightwatch which makes
development of tests easier and faster.

Example:
`grunt nightwatch --file="createsDatabase"` launches the
createsDatabase.js testfile in app/databases/tests/nightwatch

The code overwrites initial command as we don't have access to
the passed options to grunt in the `initConfig` setter. Another
solution could be using `process.argv` - however I am not a
Grunt expert and would like to get some feedback which approach
is the better way to write Grunt 

For me using `process.argv` would be the way to go (I realized I 
could probably access it from here when I finished the other 
code) but maybe it is a grunt-antipattern for reasons I am not 
aware of

Happy testing!