Pass dataset name as variable to snapshot-cmd post-snapshot-cmd

[chrisb86]

It would be really helpfull if we could pass the name of the dataset in question to the pre and post snapshot cmds.

So we could use scripts that make decisions that are related to the actual data that should be backed up. I have a script that is checking if the disks are in standby and prevents to wake them up every 15 minutes just to make a snapshot with no changes. Another use case would be shutting down databases before taking a snapshot.