RFC: partial model validation

[basz]

To be able to partially validate models, I have added a file with this content - which overrides the BsForm provided by this package.

Setting `{{bs-form validationOptions=(hash on=(array 'fieldPath') excludes=(array 'otherPath')) enables this.

I'm not sure about model.set('validations.errors', validations.get('errors')); but model.get('isTruelyValid') seems to work.

import ObjectProxy from '@ember/object/proxy';
import { Promise as EmberPromise } from 'rsvp';
import BsForm from 'ember-bootstrap/components/bs-form';

export default BsForm.extend({
  validationOptions: {},
  validate(model) {
    return new EmberPromise((resolve, reject) => {
      let m = model;

      if (model instanceof ObjectProxy) {
        m = model.get('content');
      }

      m.validate(this.get('validationOptions')).then(
        validations => {
          model.set('validations.errors', validations.get('errors'));
          model.get('validations.isTruelyValid') ? resolve() : reject();
        },
        error => {
          reject();
        }
      );
    });
  }
});

Thoughts? think it might be good to create a PR?