code: function addFormat(name, format) {
    if (typeof format == 'string') format = new RegExp(format);
    this._formats[name] = format;
    return this;
  }
docs: Add custom format
  @this   Ajv
  @param {String} name format name
  @param {String|RegExp|Function} format string is converted to RegExp; function should return boolean (true when valid)
  @return {Ajv} this for method chaining