function getCallback(func, thisArg, argCount) {
      var result = lodash.callback || callback;
      result = result === callback ? baseCallback : result;
      return argCount ? result(func, thisArg, argCount) : result;
    }
Gets the appropriate "callback" function. If the `_.callback` method is
customized this function returns the custom method, otherwise it returns
the `baseCallback` function. If arguments are provided the chosen function
is invoked with them and its result is returned.

@private
@returns {Function} Returns the chosen function or its result.