Add arch support for f-droid

[BarbossHack]

Issue is described here : #192

It is now possible to download a specific variant of an app with the arch= option:

apkeep -a [email protected] -d f-droid -o arch=arm64-v8a .

Fixes #192

[Hainish]

Thanks for the contribution! PR looks good, I'm inclined to merge as is. One question: I'm curious about the design decision to make this an option which applies to all apps on a given run, rather than a per-app specification like e.g. [email protected]@arm64-v8a. I could see some utility in, for instance, downloading a set of apps in one architecture and another set of apps in a different architecture on the same run.