- Changes "container" for m4a and m4b to "mp4" as that is
the official container type.
- Changes "extension" checks for m4b to "container" checks
which check for "mp4". This makes it possible to produce
chaptered m4a files as well.
- Needs awk, but I think portable awk code is used. This
should probably be tested on other platforms, like macOS.
- Adds a check for mp4chaps, but that should be included
in the same package as mp4art.
- Only adds chapter data to mp4 files if using single-file
output mode.
