Allow adding custom transform filters

`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!