Support Image sx, sy, etc

I would really like the ability to be able to specify all the possible arguments to the [CanvasRenderingContext2D.drawImage()](https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/drawImage) function.

I am drawing an image as a background to a canvas, and want to give the user the ability to position the background image in any way they please.

As I look through the source code, it look like a possibility if a user is able to set [`options.focusPoint`](https://github.com/Flipboard/react-canvas/blob/master/lib/CanvasUtils.js#L44) in `CanvasUtils.drawImage`. However, the only place in code that calls `CanvasUtils.drawImage` doesn't even [specify the `options` parameter](https://github.com/Flipboard/react-canvas/blob/04efecc13d8f8708a090c50b603a1105117658de/lib/DrawingUtils.js#L381), so it will always remain undefined no matter what I do.

It would be nice to be able to specify this option and have the options fall through to the `ctx.drawImage()`. Something like

``

or maybe be able to put it in the style attributes