Gunicorn, single mp3 handling, m4a handling, fixes

Features:

• CLI: Add case to handle single mp3 inputs, closes #18
• CLI: Add case to handle single m4a files.
• Web: Switch to gunicorn for the webserver.
• CLI: Losslessly merge m4b files instead of converting them.
• CLI: Move processed folders to a done folder.

Enhancements:

• Docker: Faster and smaller builds by reducing ffmpeg modules.
• Code optimizations.
• CLI: Set logging level of m4b-tool depending upon log_level
• CLI: Use m4b-tool verbose mode.

Fixes:

• CLI: Fix some path in_dir.parent sections (this needs to be tested before merge).
• CLI: Don't blindly trust that input exists.
• CLI: Replace double quotes in metadata with single quotes to avoid command issues.
• CLI: Fix multi-disc.

Documentation:

• Fix a few typos in the docker run command
• Change documentation for gunicorn commands
• Add CLI help output.