code: function registerPropertyType (type, defaultValue, parse, stringify) {
    if ('type' in propertyTypes) {
      error('Property type ' + type + ' is already registered.');
      return;
    }
  
    propertyTypes[type] = {
      default: defaultValue,
      parse: parse || defaultParse,
      stringify: stringify || defaultStringify
    };
  }
docs: Register a parser for re-use such that when someone uses `type` in the schema,
  `schema.process` will set the property `parse` and `stringify`.
  
  @param {string} type - Type name.
  @param [defaultValue=null] -
  Default value to use if component does not define default value.
  @param {function} [parse=defaultParse] - Parse string function.
  @param {function} [stringify=defaultStringify] - Stringify to DOM function.