Add per-version flags
This implements a per-version "flags" fieldlike the one discussed in https://gitlab.com/fdroid/fdroiddata/issues/169. This will allow us to mimic anti-features/features like in http://www.freshports.org/ports-mgmt/pkg-devel/.
I don't know if this is done correctly, of if it's even desired. It's just something to start the discussion. We will want to add validations later.
Also note: I removed a trailing comma in the last item of the list. I hope this is correct.