Added support for custom validation methods.
Added support for custom validation methods.

The data-* attributes are:
- data-validator (classname, method)
- data-validator-message  (message)

*classname* is a string which must be set as a class on the element to
validate.
*method* is a string denoting the name of the validating function. If it's
a nested/namespaced method, you can provide dot notation.
*message* is the error message which will be returned if validation fails.

Patternslib/require-experimental-build