Allow codadd to create .codadef files

[svniemeijer]

We currently have a shell script that uses grep/set/sort/head to determine the last modification date and then uses zip to create the .codadef file (from a directory of .xml files).
The problem is that this script only works on Linux/macOS systems but not on Windows.

It would be more convenient to have this functionality provided by the codadd tool:

1. it would allow us to build .codadef files on Windows
2. other software that wants to create .codadef files can then use a tool that already comes installed with CODA (instead of having to bundle their own version of a codadef.sh script).