OPDS: let user choose download path

This should fix #882
I think we could easily wrap the defaults.lua in a way that allows us to make those variables local.
Global variables are a problem because they clutter the namespace in a non-transparent way. Sure, we have naming conventions in place that will probably save us from clashes - for now. But then, it would be easy to make this configuration local. I put the architectural argument first. There is also the point that global variables can (will?) prevent code from getting JIT-compiled. Which isn't nice either.