Add top-level option for mountOptions

[aftix]

In impermanence, there's a top-level hideMounts option that will add x-gvfs-hide as a mount option to all bind mounts. Currently, I have a simple mkFiles and mkDirs function that adds this bind mount to every item in files = or directories = list, but it would be nice to be able to set mount options like these at the preserveAt and preserveAt.<path> levels.