code: function(rule, fn) {
        if (is_object(rule)) {
          _replacers = extend({}, _replacers, rule);
        } else if (is_string(rule)) {
          _replacers[rule] = fn;
        }
      }
docs: add message replacers
  
  @param {String/Object} rule
  @param {Function}      fn
  
  @return {Void}