Enhancement: allow overriding of DataSheet events (onMouseOver, etc.)

[kylegoetz]

I want to be able to override DataSheet events but don't see a way to do this currently. In particular, I want to disable the default behavior for when you click and drag across multiple cells (basically I want to have it just highlight the cell you clicked and that's it: I have the grid in a draggable div and the two drags conflict with one another).

In the constructor for DataSheet it's as easy as _this.onMouseOver = (_this.props.onMouseOver || _this.onMouseOver).bind(this) for instance, and then if you do <ReactDataSheet onMouseOver={myFunction}/> then whatever myFunction is will override the default behavior.

I think I'm going to go ahead and implement this in my fork soon because it's something I need for a project I'm working on. I'll create a PR from there and if this isn't a desired feature or my implementation sucks, feel free to reject the PR or suggest a different approach and I might have time to do that myself.