Graph Inserts and Upserts

[NotAmaan]

Since graph upsert and insert is a major and rather useful feature of objection.js, I think this module should also offer support for it.

I have it partially implemented in a privately maintained fork. I'll migrate the basic functionality over soon.

[Aerlin]

The error message when not setting allowedInsert properly isn't very informative.

error: BadRequest: Error
    at new BadRequest (/project_path/node_modules/@feathersjs/errors/lib/index.js:86:17)
    at errorHandler (/project_path/node_modules/feathers-objection/lib/error-handler.js:48:25)
    at tryCatcher (/project_path/node_modules/bluebird/js/release/util.js:16:23)
    at Promise._settlePromiseFromHandler (/project_path/node_modules/bluebird/js/release/promise.js:512:31)
    at Promise._settlePromise (/project_path/node_modules/bluebird/js/release/promise.js:569:18)
    at Promise._settlePromise0 (/project_path/node_modules/bluebird/js/release/promise.js:614:10)
    at Promise._settlePromises (/project_path/node_modules/bluebird/js/release/promise.js:689:18)
    at Async._drainQueue (/project_path/node_modules/bluebird/js/release/async.js:133:16)
    at Async._drainQueues (/project_path/node_modules/bluebird/js/release/async.js:143:10)
    at Immediate.Async.drainQueues (/project_path/node_modules/bluebird/js/release/async.js:17:14)
    at runCallback (timers.js:672:20)
    at tryOnImmediate (timers.js:645:5)
    at processImmediate [as _immediateCallback] (timers.js:617:5)

Took me a moment to realize I hadn't updated allowedInsert when adding additional relationships to a model.

Anyway great work on the pull request. :)