scrollbar stays at the top when infinite scrolling

hey, thanks for the great package!

i'm using infinite scrolling on the `MessageList` component like this:
```

```

but when more messages are loaded, the scrollbar is still at the top of the container so it just keeps calling `loadMore` over and over. ideally the behaviour would be something like this:

before onYReachStart is called:
```
message4 <-----scroll to here
message5
message6
```

after onYReachStart is called:
```
message1
message2
message3
message4 <-----scroll bar is here
message5
message6
```

so that only the next page of messages is fetched, rather than continually fetching more pages until you manually scroll down. hope that makes sense!