formField(name, options = {}) {
    let fieldDict = this._fieldDict(name, null, options);
    let fieldRef = this.ref(fieldDict);
    this._addToParent(fieldRef);
    return fieldRef;
  }
Creates and adds a form field to the document. Form fields are intermediate
nodes in a PDF form that are used to specify form name heirarchy and form
value defaults.
@param {string} name - field name (T attribute in field dictionary)
@param {object} options  - other attributes to include in field dictionary