Allow custom formats in JSON Schemas

Currently, all subclasses of SimpleTypeSchema implement the format keyword of a Json Schema using a JsonValueFormat. But, according to the specification (http://json-schema.org/latest/json-schema-validation.html#anchor106), which is also helpfully copied into ValueTypeSchema:

> Additional custom formats MAY be created.

Since JsonValueFormat is an enum this is impossible. It would be very useful, especially on StringSchema, to allow custom formats.

It would be great if JsonValueFormat could be replaced with a String, or a normal class with maybe default static implementations.