Support options for `ajv`

[AlexAxthelm]

Hi,

I'm interested in using jsonvalidate for my projects, and one of the features that would be really helpful from ajv is being able to fill in default values from the schema.

Doing so would require being able to set the useDefaults option for ajv. Is there a way to pass options other than strict to json_schema$new() without using a modified version of in.js?