Add onLoad to the available props.

[soorajvnair]

Hey there, Great job on the library, I think it's useful to have onLoad property available, so that we're aware when the image has loaded, in my use-case i want to render couple of components only when the image has finished loading, currently that's not possible since the onLoad event is not available on the component.

i think there's already a great PR by @hosmelq [https://github.com/nitish24p/react-worker-image/pull/16](Let user pass any attributes) about letting the user pass all available image properties to the container, if you guys decide to merge that PR then i think my use-case would be covered.

I have also used prettier to format the code, i think that keeps in line with the standard these days, where most libraries are using prettier to format the source code.

Once again, great job on the library. Well done. :)