Add support for external references

[BryanHunt]

I have several use-cases where a resource has a reference to a resource managed by another server. I can get the reference to work by defining a dummy resource, but it would be nice if there were built in support for this. Maybe something like:

owner: jsonApi.Joi.one('users').external()

[pmcnr-hx]

That sound like a very reasonable extension @BryanHunt. We'll be happy to merge support for it if you submit a PR.