Add a command line arg to disable update checks

[C-512L]

I'm working on a flatpak package for Digital and one thing i noticed is the code for CheckForNewRelease.java which checks for new releases using the GitHub API.

Since flatpak in this case takes care of keeping Digital updated, it would be nice to have a command line option (i.e:-Ddig.rev.disable=true) to disable the update checks.