Add support for width/height for screenshots

[samunderwood]

It'd be useful if you could use width/height parameters for the screenshot functionality - https://statically.io/docs/using-screenshot/ - Like you can do for images.

Also, an option to deliver with WebP would be great!

[PeterDaveHello]

From the SCREENSHOT tab on the homepage, the tips seem to indicate that it's supported already:

Image resizing w and h available only on desktop mode.

but the whole screenshot feature is not working right now: https://github.com/staticallyio/screenshot/issues/10, you may want to setup your own instance to test it.