Honour --gruntfile argument

[waiyan1612]

Currently, we cannot use this plugin to work with a grunt file with a custom name since --gruntfile is hardcoded in GruntTask.groovy. It'd be great if this can be parameterized.

[srs]

Yes, that would be a good option. Suggest that we add a gruntFile setting to the grunt-extension. Example:

grunt {
  gruntFile = 'grunt.js'     // default to 'Gruntfile.js
}