Add Audio Folder
Would resolve  #3964

AudioFolder loads a .txt file with transcriptions and creates a dataset with all audiofiles in provided directory that has a transcription (independently of the directory structure) as a single split (train).

Can be loaded via:
```python
# for local dirs
dataset = load_dataset("audiofolder", data_dir="/path/to/folder", transcripts_filename="transcripts.txt")
```
```python
# for local and remote zip archives
dataset = load_dataset("audiofolder", data_files="path/to/archive/archive.zip", transcripts_filename="transcripts.txt")
```
default transcriptions filename is `transcripts.txt`. it should have the following structure:
```
audio_id_1   transcription text 1
audio_id_1   transcription text 1
```
separator is `\t`!

---
sorry for first old commits from other branch, don't know how that happened... 