Add option to declare mount point

[Sylvan86]

If you want to call snap-sync automatically e.g. via a cronjob you can do this with the parameters "--UUID", "--subvolid" and "--noconfirm".
However, if there are multiple mount points for this volume (e.g. for an nfs bind mount), this method fails because user interaction is required.

Therefore I suggest to implement an additional parameter to define the desired mount point.