Use grunt.util.spawn to execute the grunt task. 

Hi,

I've ran into problems when executing tasks with grunt configuration specified via `grunt.config`, e.g:

    grunt.config('jade', {
       options: {
          pretty: true
       },
       files: [...]
    });

I solved it by using `grunt.util.spawn` to execute the grunt task. This makes sure grunt configuration set via grunt.option is properly read again.