Add support for transcribing all files in a directory recursively.
The implementation lets ffmpeg handle file validation instead of
filtering by extension. Update README with minimal documentation
for directory support.
* add support for audio and input name from stdin
* refactored to stdin - arg, and output-name template
* fix bugs, add test coverage
* fix doc to match arg rename
* some nits
---------
Co-authored-by: Awni Hannun <awni@apple.com>
