nightly: new command to setup nightly builds in CI systems

This lets people using CI systems easily setup a nightly build repo hosted
for free in github or gitlab.

https://github.com/zom/Zom-Android/pull/355
https://gitlab.com/fdroid/fdroidclient/merge_requests/594

environment variables:
* https://docs.gitlab.com/ce/ci/variables/README.html
* https://docs.travis-ci.com/user/environment-variables
* https://circleci.com/docs/1.0/environment-variables

fdroid

@@ -47,6 +47,7 @@ commands = OrderedDict([
     ("signindex", _("Sign indexes created using update --nosign")),
     ("btlog", _("Update the binary transparency log for a URL")),
     ("signatures", _("Extract signatures from APKs")),
+    ("nightly", _("Set up an app build for a nightly build repo")),
 ])