v1.3 - audible-cli Integration

This release allows experienced users to use audible-cli derived files to generate the various outputs that this script supports.

For example:

audible download --aaxc --cover --cover-size 1215 --chapter --all
bash AAXtoMP3 -e:m4b -c -D 'Audiobooks/$title \($narrator\)' --use-audible-cli-data *.aaxc

Also in this release macOS 11.5.2 has been confirmed as working with updated checks in the script to check for GNU versions of sed, find, and grep.

Many thanks to @fabh2o for continuing to push the capabilities of this script forward along with all of the other contributors for their work.