feat(ngModelOptions): Model update behavior can now be customized

By default, any change on the content will trigger an immediate model update and form validation. This PR implements a new directive `ng-model-options` that allow you to override this default behavior in several ways.

You should specify an object with the different parameters.

For example, it allows to trigger an update only when a particular event or list of events is received by the input using the `updateOn` key. Should you need multiple events, just use a space delimited string.

I.e. `ng-model-options="{ updateOn: 'blur' }"` will update and validate only after the control loses focus.

If you want to keep the default behavior and just add new events that may trigger the model update and validation, add "default" as one of the specified events.

I.e. `ng-model-options="{ updateOn: 'default submit' }"`

Also, with the `debounce` option, `ng-model-options` will allow deferring the actual model update until a timer expires. The timer will be reset each time an event is triggered.

I.e. `ng-model-options="{ debounce: 500 }" for 500ms after the latest event.

Custom timeouts for each event can be set for each event if you use an object in `debounce`. This can be useful to force immediate updates on some specific circumstances (like blur events).

I.e. `ng-model-options="{ updateOn: 'default blur', debounce: { default: 500, blur: 0} }"`

You can use the directive in any tag so its contents became the default settings for any child control, although they can be overridden.

Closes #1285, #2129