`diff` subcommand for `snapshot`

[Bloodchiefy]

What it does?

This PR adds a diff subcommand for snapshot. This enables the user two show the difference between two snapshots. The proposed table from #224 couldn't be completely satisfied, since moving / renaming files is not easy to track (if it is though, please let me know).

An example output looks like this:

$ knoxite -r /tmp/backup snapshot diff eb63a418 aa159abb

Snapshot (eb63a418)                               Snapshot (aa159abb)                               Operation  User      Group  Perms     
------------------------------------------------------------------------------------------------------------------------------------------
/home/usern/Documents/backup_test/test/test1.txt  /home/usern/Documents/backup_test/test/test1.txt  Unchanged  usern     1000   -rw-r--r--
-                                                 /home/usern/Documents/backup_test/test/test3.txt  Created    usern     1000   -rw-r--r--
/home/usern/Documents/backup_test/test1.txt       /home/usern/Documents/backup_test/test1.txt       Modified   usern     1000   -rw-r--r--
/home/usern/Documents/backup_test/test2.txt       /home/usern/Documents/backup_test/test2.txt       Unchanged  usern     1000   -rw-r--r--
/home/usern/Documents/backup_test/test4.txt       /home/usern/Documents/backup_test/test4.txt       Unchanged  usern     1000   -rw-r--r--
/home/usern/Documents/backup_test                 /home/usern/Documents/backup_test                 Modified   usern     1000   drwxr-xr-x
/home/usern/Documents/backup_test/test            /home/usern/Documents/backup_test/test            Modified   usern     1000   drwxr-xr-x
/home/usern/Documents/backup_test/test3.txt       -                                                 Deleted    usern     1000   -rw-r--r--