Automatically download 3D models

[corporateshark]

• Automatically download the Khronos 3D models repository (via https://github.com/corporateshark/bootstrapping).
• Load the Sponza model if nothing is specified on the command line.

[theWatchmen]

Just to confirm, with this change users will have to run git submodule update --init before being able to run the bootstrap script?

[corporateshark]

Right. Or they can clone the entire repo just like this right from the get-go:

git clone --recurse-submodules https://github.com/PacktPublishing/Mastering-Graphics-Programming-with-Vulkan

[theWatchmen]

Cool. We'll wait to merge this until after the release of the book. I know most people will be able to figure it out, but this change would create a divergence with the instructions in the book and we'd like to avoid confusion :)

[corporateshark]

I think this solves the issue #4

[corporateshark]

@theWatchmen it looks like there's some strong demand for this.

[theWatchmen]

Thanks for this Sergey! I updated the README to include the clone instructions :)