allow dash's (-) in snapshot_prefix and/or snapshot_label

[Andrey7-b]

Can support for dash be added for snapshot_prefix and snapshot_label, or it create any issues?
If it is ok, the following line can be modified
432:# Allow only letters, numbers and underscores for the snapshot prefix.
433:[[ ! $snapshot_prefix =~ ^[A-Za-z0-9_]+$ ]] && die "The snapshot prefix may only contain letters, digits and underscores."
434:
435:# Allow only letters, numbers and underscores for the snapshot label.
436:[ -v snapshot_label ] && [[ ! $snapshot_label =~ ^[A-Za-z0-9_]+$ ]] && die "The snapshot label may only contain letters, digits and underscores."

to:

432:# Allow only letters, numbers, dashes and underscores for the snapshot prefix.
433:[[ ! $snapshot_prefix =~ ^[A-Za-z0-9_-]+$ ]] && die "The snapshot prefix may only contain letters, digits, dashes and underscores."
434:
435:# Allow only letters, numbers, dashes and underscores for the snapshot label.
436:[ -v snapshot_label ] && [[ ! $snapshot_label =~ ^[A-Za-z0-9_-]+$ ]] && die "The snapshot label may only contain letters, digits, dashes and underscores."

[Andrey7-b]

changes added into pull request #44