Merge branch 'hotfix-fix121'
Added support for subvolume snap shotting

In the current master branch btrfs-snap will not create a backup of a nested subvolume. The proposed changes add support for subvolumes by querying for subvolumes located in the specified mountpoint. If `btrfs subvolume list ${mp$}` returns an entry which matches the end of `${mp}` it is a valid subvolume.
Switched subvolume detection method

Switched subvolume detection method to use `btrfs subvolume show`. This is a valid method on btrfs-tools v0.20-rc1 or greater.
Merge branch 'feature/support_subvolume_snapshot' of https://github.com/bkloppenborg/btrfs-snap into feature-unmounted-subvolumes
Merge branch 'feature-unmounted-subvolumes' into develop