function onResult(result) {
    result = result === undefined ? 0 : result;
    opt.callback(null, stdout);
    return stdout;
  }
Simple binding interface between
each command's function and the caller
of the command, whether by `cash.command`
or by being invoked by Vorpal.
This exists to abstract complexity
and create a standard in interfacing
commands acorss muliple execution paths.

@param {Object} opt
@api public