adding karmaExecutable as a parameter

The plug-in currently only supports karma being installed globally. In order to support karma being installed in the local node_modules there needs to be an option to pass in the path to the executable. I know the karma documentation says you should install it globally, but that isn't always an option. 
Here is how you'd add the parameter to the plugin configuration:
<karmaExecutable>${project.basedir}/node_modules/.bin/karma</karmaExecutable>