Pass custom metadata filename to Image/Audio folders
This is a quick fix. 
Now it requires to pass data via `data_files` parameters and include a required metadata file there and pass its filename as `metadata_filename` parameter.
For example, with the structure like:
```
data
   images_dir/
      im1.jpg
      im2.jpg
      ...
   metadata_dir/
      meta_file1.jsonl
      meta_file2.jsonl
      ...
```
to load data with `metadata_file1.jsonl` do:
```python
ds = load_dataset("imagefolder", data_files=["data/images_dir/**", "data/metadata_dir/meta_file1.jsonl"], metadata_filename="meta_file1.jsonl")
```

Note that if you have multiple splits, metadata file should be specified in each of them in `data_files`, smth like:
```python
data_files={
"train": ["data/train/**", "data/metadata_dir/meta_file1.jsonl"], 
"test": ["data/train/**", "data/metadata_dir/meta_file1.jsonl"]
}
```