Validator::Custom validate HTML form data easy and flexibly. The features are
the following ones.
- Many constraint functions are available by default, such as not_blank, int,
  defined, in_array, length.
- Several filter functions are available by default, such as trim,
  datetime_to_timepiece, date_to_timepiece.
- You can register your constraint function.
- You can set error messages for invalid parameter value. The order of messages
  is keeped.
- Support OR condtion constraint and negativate constraint.