Allow command function to accept arguments

I ran into a situation where it was useful to create a command string based on arguments, e.g.:

    shell: {
      someTask: {
        command: function(myArg){
          return 'command ' + myArg;
        }
      }
    }

    # grunt shell:someTask:foo

Which would run <code>command foo</code>

I anticipate such a feature may be useful for others as well (and selfishly I'd like this feature to be in the npm package version ;)