Make onYReachStart margin based

[KDederichs]

Hey,

as described here https://github.com/chatscope/chat-ui-kit-react/issues/78 you'll run into issues with infinite scroll cause once you're at the top the scroll bar will just stay there and cause infinite history loading till there's no more.

I'd propose an optional prop where you can pass a value (hard value or percent) to the MessageList and after the user passes that point the callback function gets called.

That way you can start loading new messages while the user is still scrolling, avoiding being stuck at the top.