Refactor model configuration structure and update README

- Changed the configuration format to use a single 'models' list with entries containing 'url' and 'type'.
- Updated validation logic to ensure 'models' entries are correctly structured.
- Modified download logic to check for existing directories before downloading.
- Revised README to reflect new configuration format and usage instructions.

Signed-off-by: Daniel Henry <iamdanhenry@gmail.com>

model_downloader/__main__.py

@@ -27,7 +27,7 @@ def main() -> int:
         "--only",
         nargs="+",
         metavar="TYPE",
-        help="Only process these model types (e.g. diffusion_models loras)",
+        help="Only process these model types (e.g. loras diffusion_models)",
     )
     args = parser.parse_args()
 
@@ -48,7 +48,7 @@ def main() -> int:
 
     tasks = get_model_tasks(config, only_types=args.only)
     if not tasks:
-        print("No model URLs to download. Add URLs under diffusion_models, text_encoders, vaes, upscale_models, or loras in your config.")
+        print("No model URLs to download. Add entries under models with url and type in your config.")
         return 0
 
     if args.dry_run: