code: function defineKeywords(ajv, keyword) {
    if (Array.isArray(keyword)) {
      for (var i=0; i<keyword.length; i++)
        get(keyword[i])(ajv);
      return ajv;
    }
    if (keyword) {
      get(keyword)(ajv);
      return ajv;
    }
    for (keyword in KEYWORDS) get(keyword)(ajv);
    return ajv;
  }
docs: Defines one or several keywords in ajv instance
  @param  {Ajv} ajv validator instance
  @param  {String|Array<String>|undefined} keyword keyword(s) to define
  @return {Ajv} ajv instance (for chaining)