Add support for initial values

[niho]

Make it possible to initialise the form with initial values to make it possible to create "edit" forms. It is currently possible to specify default values in the schema, but those are not useful if the form has already been submitted with user supplied values. If that is the case you might want to show the form again, but this time with the user supplied values as default values.

Example:

Json.Schema.Form.init options schema initialValues