lsblk: add option --tree

lsblk: add option --tree

Now lsblk uses --list when --sort <column> is specified. This patch
allows to specify --tree to overwrite this default behavior add to
force tree-like output. In this case tree branches are sorted by the
<column>.

$ lsblk --sort SIZE
NAME MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda2   8:2    0   200M  0 part /boot
sda1   8:1    0   200M  0 part /boot/efi
sda6   8:6    0   7.8G  0 part [SWAP]
sda5   8:5    0  35.1G  0 part /home/misc
sda4   8:4    0    50G  0 part /
sdb1   8:17   0  74.5G  0 part /home/archive
sdb    8:16   0  74.5G  0 disk
sda3   8:3    0 130.3G  0 part /home
sda    8:0    0 223.6G  0 disk

$ lsblk --sort SIZE --tree
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sdb      8:16   0  74.5G  0 disk
└─sdb1   8:17   0  74.5G  0 part /home/archive
sda      8:0    0 223.6G  0 disk
├─sda2   8:2    0   200M  0 part /boot
├─sda1   8:1    0   200M  0 part /boot/efi
├─sda6   8:6    0   7.8G  0 part [SWAP]
├─sda5   8:5    0  35.1G  0 part /home/misc
├─sda4   8:4    0    50G  0 part /
└─sda3   8:3    0 130.3G  0 part /home

Signed-off-by: Karel Zak <kzak@redhat.com>