override protractorMainPath

[marneborn]

I would like to always use the protractor that I have installed, instead of the one that comes with this task.
I noticed because I'm testing protractor 4.0, and grunt-protractor-runner uses "^3.2.0".

I know that I could rm -rf node_modules/grunt-protractor-runner/node_modules/protractor.
But counting everyone doing that on every new install isn't a great idea.

Instead I would like to push a pointer to protractor through the options.
Something like this:

grunt.initConfig({
  protractor: {
    options: {
      protractorMainPath: require.resolve('protractor')
    }
  }
});

What do you think?