add an argument that lets user specify folders to scan for weights (#1977)

* add an argument that lets user specify folders to scan for weights

This PR adds a `--weight_folders` argument to invoke.py. Using
argparse, it adds a "weight_folders" attribute to the Args object, and
can be used like this:

```
'''test.py'''
from ldm.invoke.args import Args

args = Args().parse_args()

for folder in args.weight_folders:
    print(folder)
```

Example output:

```
python test.py --weight_folders /tmp/weights /home/fred/invokeai/weights "./my folder with spaces/weight files"
/tmp/weights
/home/fred/invokeai/weights
./my folder with spaces/weight files
```

* change --weight_folders to --weight_dirs

ldm/invoke/args.py

@@ -431,6 +431,12 @@ class Args(object):
             '--model',
             help='Indicates which diffusion model to load (defaults to "default" stanza in configs/models.yaml)',
         )
+        model_group.add_argument(
+            '--weight_dirs',
+            nargs='+',
+            type=str,
+            help='List of one or more directories that will be auto-scanned for new model weights to import',
+        )
         model_group.add_argument(
             '--png_compression','-z',
             type=int,