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
  };
}
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.