Allow adding of new items to BLTNItemManager instance without pushing immediately

### New Issue Checklist

- [x] I updated BulletinBoard to the latest version.
- [x] I read the [Contribution Guidelines](https://github.com/alexaubry/BulletinBoard/blob/master/CONTRIBUTING.md).
- [x] I read the [documentation](https://alexaubry.github.io/BulletinBoard).
- [x] I searched for [existing GitHub issues](https://github.com/alexaubry/BulletinBoard/issues).

### Issue Description

Current BulletinBoard build requires us to declare the manager instance at the start of the view controller. However, at the start of the program, my app has not received any data yet. I can only pass in the data to show in the BulletinItem after user performs a certain action.

At the moment I can only add a new item to the bulletinManager through the `manager?.push(item:)` method. This method forces the bulletin to **immediately** change to the new item that was pushed in.

This behaviour may be mitigated by,
```
page.nextItem = makeLocationPage() // Creates a new PageBulletinItem

page.actionHandler = { (item: PageBulletinItem) in
item.manager?.displayNextItem()
}
```
but as my root bulletin item **DOES NOT** contain the params that I need at the start of the app, I can't pass them in when I first declare the root bulletin item in my bulletinManager.

Best solution now is to allow the manager to have a method of adding a BLTNPageItem without immediately presenting the new item.

### Environment

- **BulletinBoard Version**: 2.0.1
- **iOS Version**: 11.4
- **Device(s)**: iPhone X