Modified grunt-jsbeautifier to work as a multi-task

Hi,

I modified the task to use `grunt.task.registerMultiTask()` so we can specify multiple named targets with cascading options just like the grunt-contrib tasks. Of course it still works with the simple command as before.

If you want to pull this modification I can add some tests as well, let me know if that's the case.

Grtz,

Bart