code: function getKeyword(keyword) {
    /* jshint validthis: true */
    var rule = this.RULES.custom[keyword];
    return rule ? rule.definition : this.RULES.keywords[keyword] || false;
  }
docs: Get keyword
  @this  Ajv
  @param {String} keyword pre-defined or custom keyword.
  @return {Object|Boolean} custom keyword definition, `true` if it is a predefined keyword, `false` otherwise.