wish: If no callback is supplied return a Promise

Node.js Now has some Promise methods in the core as well as `async` and `await`.

Having `validate` return a promise if no callback is provided would a welcome addition.

Bluebird's `asCallback()` method as one of example of supporting both callback and Promise APIs.

http://bluebirdjs.com/docs/api/ascallback.html

Start at the `validate` API and inspect how it currently handles callback arguments. Add coverage for calling `validate` without a callback and verify that it returns a Promise while the existing callback behavior remains supported.

Written by the indexing model from the issue text.