Feature request: Allow specifying a command-line argument for MUTAGEN_DATA_DIRECTORY

[rfay]

In DDEV we specify a private MUTAGEN_DATA_DIRECTORY.

However, environment variables are so very global and very difficult to control at the proper level.

I'd much rather be able to do mutagen --data-directory=<dir> <args> so we'd know that we're completely controlling the context that mutagen is running in, without side-effects from environment variables that might be set wrong.

[xenoscopic]

Very reasonable ask. I'll try to get this into the next Mutagen work sprint.