fix: Pass node-specific command line params to the subprocess

This commit makes the karma task pass Node-specific parameters used to start
the Node process to its background subprocess. This is required e.g. to be able
to use the --harmony flag.

Similar pull request for Grunt itself (already merged):
https://github.com/gruntjs/grunt/pull/877
Merge pull request #117 from mzgol/node-args

fix: Pass node-specific command line params to the subprocess