Add option for absolute file paths in the output formatters.

The formatters from csslint use relative file names in the generated reports. In some cases the tools which consume theses reports need absolute file names (i.e. Violations Plugin in Jenkins). 
So now you can set the option ```absoluteFilePathsForFormatters: true``` in the ```Gruntfile.js``` so that in result all the file names in the reports will be absolute.