Implement the `--dry-run` flag

[sharpvik]

sema is a powerful tool with a lot of flags in place. They combine in a lot of ways too. I would like to know exactly what will happen after a command without any changes taking place to confirm that I'm doing the right thing.

[sharpvik]

For this, the --dry-run | -d flag can be added. It will run sema, avoiding any effectful changes, merely describing what would happen.