lsblk: add --sort <column>

lsblk: add --sort <column>

This implementation allow to sort output by arbitrary output column.

Note that output is always converted to human readable strings, but
sort functions in many cases work with original data (stored as
private data to cells by scols_cell_set_userdata()).

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