Configure platforms for multiarch globally

[andyjeffries]

On the webinar I saw that you can specify --platform= to build multiarch, but it would be really nice if you could list them in an .acornrc in your home folder or something so it always does it without needing to keep specifying those params every time.

[ibuildthecloud]

This is a good idea. This would be the first need for a config file, but I'm sure there are more to come.