feat: add --stats

[chapmanjacobd]

looks like this:

$ sudo btrfs subvolume snapshot -r ~/d/ ~/d/.snapshots/two
$ sudo btrfs send --no-data -p ~/d/.snapshots/one/ /mnt/d/.snapshots/two/ > /tmp/diff_file
$ ./bin/btrfs-snapshots-diff-summary.py -f /tmp/diff_file --stats
Commands:
1790787	utimes
1546893	set_xattr
520245	unlink
497927	link
69900	update_extent
30723	rename
27325	chown
27325	chmod
22684	mkfile
8064	rmdir
4641	mkdir
8	remove_xattr
3	truncate
1	snapshot
1	end