Add new option to disable 'gzip' report generation

Using grunt-lib-contrib to report the minified/gzipped size of the compressed files is useful, but I'd like to be able to disable it for performance reasons (it can slow the task by 10x, as noted in [the lib](https://npmjs.org/package/grunt-lib-contrib))

The API is identical to [grunt-contrib-cssmin's report option](https://github.com/gruntjs/grunt-contrib-cssmin#report), and I've updated the `cssmin` task to show how it can be used.