btrup: Simple but fast btrfs-based backups in Rust

Implements https://btrfs.wiki.kernel.org/index.php/Incremental_Backup.

Requirements: - root fs with btrfs - usb disk with btrfs - btrfs-tools

``` Usage: btrup [options] btrup -h | --help

Options:
  -h, --help      Show this message.
  -p, --prune     Prune old local snapshots.

Example:
  sudo btrup -p /mnt/usbdisk/backup

```