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

completion/bash-completion

@@ -264,6 +264,12 @@ __complete_btlog() {
 	__complete_options
 }
 
+__complete_nightly() {
+	opts="-v -q"
+	lopts="--show-secret-var"
+	__complete_options
+}
+
 __complete_stats() {
 	opts="-v -q -d"
 	lopts="--verbose --quiet --download"
@@ -310,6 +316,7 @@ import \
 init \
 install \
 lint \
+nightly \
 publish \
 readmeta \
 rewritemeta \