Backslashes in `--with` aren't sanitized when saved

Example: 
```sh
source set_data_dir.sh tests/data
psc-plot test-2d/pfd ey_ec -v y z loc=1,1 --with test-3d/pfd::ey_ec -v y z loc=1,2 -qs
```

Similarly, `:` tricks ffmpeg into thinking there's a protocol when there isn't.