Compatibility with Ubuntu

[damian-buho]

Hi, happy to see an alternative to antique apt-cacher-ng.

To make it work on Ubuntu, would be cool to have:

1. Support for native path like "$proxy_host/ubuntu $ubuntu-codename", we past the time when Ubuntu was kinda synced with stable/testing.
2. Document (may be in UI), where we can get GPG key, it's not enough to just add sources nowadays.
3. Suggest deb822 instead of .list in UI?

P.S. I am going to dive in source code myself and try to make a PR, I really like the project, but I am a noob in Go.

[kpfleming]

The first part of that is happening in #56, where the 'debian' protocol handler can be used for multiple Debian-style upstreams.