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

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

fixes: #887

tests/test_cli/data/plugins2/plugin2.py

@@ -0,0 +1,5 @@
+from koji.plugin import export_cli
+
+@export_cli
+def foo5():
+    pass

tests/test_cli/data/plugins2/plugin3.py

@@ -0,0 +1,7 @@
+from koji.plugin import export_cli, export_as
+
+@export_as('foo6')
+@export_cli
+def foo():
+    pass
+