Expose disabled validator option

[offirgolan]

Resolves #104

• Exposes disabled option for validators
• Add functionality to define dependentKeys in default options (should have already been there)

This will allow to do the following

// Via default options
const Validations = buildValidations({
  username: {
    debounce: 500,
    disabled() { return !this.get('model._enableValidations'); },
    dependentKeys: ['_enableValidations'],
    validators: [
      validator('presence', true),
      validator('length', {
        max: 15
      })
    ]
  }
});

// Via single validator
const Validations = buildValidations({
  username: validator('presence', {
    presence: true,
    disabled() { return !this.get('model._enableValidations'); },
    dependentKeys: ['_enableValidations']
  })
});

export default DS.Model.extend(Validations, {
  'username': attr('string'),
  _enableValidations: true
});

@stefanpenner @rwjblue open for comments.

[offirgolan]

ready for review 😺