cli: also load plugins from ~/.koji/plugins

- add plugin_path in koji.conf
- add `--plugin-paths` in cli arguments

fixes: #887

cli/koji.conf

@@ -37,6 +37,11 @@
 ;certificate of the CA that issued the HTTP server certificate
 ;serverca = ~/.koji/serverca.crt
 
+;plugin paths, separated by ':' as the same as the shell's PATH
+;~/.koji/plugins and koji_cli_plugins module are always be appended
+;plugin_paths = ~/.koji/plugins
+
+;[not_implemented_yet]
 ;enabled plugins for CLI, runroot and save_failed_tree are available
 ;plugins =