Ffmpeg-normalize

* Code formatting.

* Extend warning for audio-only format to opus, fixes 184.

* Update python requirements.

* Prevent race condition in output dir creation.

* Increase possible loudness range target to 50.

* Add way to force dynamic mode, clarify usage, fixes 176.

* Add warning for cover art, addresses 174 and 175.

* Update README.

* Improve issue templates.

* Do not print ffmpeg progress in debug logs.

* Remove unused import.

* Replace which() function with shlex version.

* Add python 3.10 in setup.py.

* Clarify minimum ffmpeg version.