Allow zip as a compression method

[nsmith5]

Describe the enhancement
zip compression would make it easy to install tools like nomad (packaged with zip from upstream). Ideally I'd like something like

files:
  - path: /usr/bin/nomad
    mode: 0644
    contents:
      compression: zip
      source: https://releases.hashicorp.com/nomad/0.11.8/nomad_0.11.8_linux_amd64.zip

to work.

[jlebon]

This looks like a dupe of coreos/ignition#867. If possible, let's keep discussions there.

(One aside: /usr/bin/nomad wouldn't work because /usr is read-only, except for /usr/local which is a symlink to /var/usrlocal. So e.g. /var/usrlocal/bin/nomad would be more appropriate.)