Exposing clearDrawing 
I've successfully exposed the clearDrawing method for external components to clear the current drawing, but before I send my pull request, I was wondering how you want to handle the current clear button? 

If we expose the clearDrawing method, the button becomes obsolete.. Maybe activate it through props?

I'm also working on an undo/redo method, but it's kind of going beyond the scope of a simple sketch component..