Ability to use multiple instances

In my opinion, this is a very important feature provided by localForage and missing in ionic-storage.

With this feature, we can iterate on different data stores, (e.g. messages in conversations)

e.g.

```
let myConversation = localForage.createInstance({name: 'conversationId'});
myConfersation.iterate((v, k) => ...);
```

https://localforage.github.io/localForage/#multiple-instances-createinstance