function getKeyword(keyword) {
  /* jshint validthis: true */
  var rule = this.RULES.custom[keyword];
  return rule ? rule.definition : this.RULES.keywords[keyword] || false;
}
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.