allow configuring additional app directories and add a writable app directory

fixes #15

src/args.rs

@@ -505,6 +505,7 @@ impl SubCommand for HazeCommand {
 fn test_arg_parse() {
     let config = HazeConfig {
         sources_root: Default::default(),
+        app_directories: Default::default(),
         work_dir: Default::default(),
         auto_setup: Default::default(),
         volume: vec![],