Merge pull request #4146 from perobertson/heroku-help

[heroku] add '--help' as an option to commands

plugins/heroku/_heroku

@@ -143,5 +143,6 @@ _arguments \
   $_command_args \
   '(--app)--app[the app name]' \
   '(--remote)--remote[the remote name]' \
+  '(--help)--help[help about the current command]' \
   &&  return 0