feature: Add onSuccess hook when any operation succeeds

I added an onSuccess hook.  My use-case is that I want to display an alert (eg 'Tenant Saved Successfully') when the form submits - whether it's an insert or an update.

Here's how I'm using it to repeat myself a little less:

```js
var tenantForm;
Meteor.startup(function() {
  tenantForm = new AutoForm(Tenants);
  tenantForm.hooks({
    onSuccess: function(type, result, temlpate) {
      if (type == 'insert') {
        Router.go('tenantDetail', {_id: result})
      }
      Alerts.success('Tenant Saved Succesfully');
    }
  });
});
```
If you think an onError callback would be a good idea too I can add it to the PR.