Allow adding custom transform filters

[JaneJeon]

dynamicDefaults allow users to add their own defaults functions by exporting DEFAULTS in the keyword, as so: https://github.com/epoberezkin/ajv-keywords/blob/master/keywords/dynamicDefaults.js#L49

However, transform does not export the transform variable - https://github.com/epoberezkin/ajv-keywords/blob/master/keywords/transform.js#L4 - and so there's literally no way for users to add custom transformations.

I think you should export the transform variable like you do in dynamicDefaults and allow people to add their own transformations, rather than bolting in other transformations like in #95 or #66.

Thanks!