Add a `-deterministic` flag to `mdoc assemble`

[joelmartinez]

In order for Mono to support a reproducible build, it needs all of its dependencies to support it as well ... including mdoc:
https://bugzilla.xamarin.com/show_bug.cgi?id=37367

mdoc's assemble subcommand should add a -deterministic flag. When this flag is present, all files should be written to the resulting .zip file with a consistent timestamp (for example DateTime.MinValue).