Add -l and --human-readable options to "bup ls".

Add -l and --human-readable options to "bup ls".

When -l is specified, include the size in bytes for each item.

When --human-readable is also specified, print sizes like 3.5K, 1.8G,
etc., instead of the exact byte count.

Signed-Off-By: Kumar Appaiah <a.kumar@alumni.iitm.ac.in>
Reviewed-by: Zoran Zaric <zz@zoranzaric.de>
[rlb@defaultvalue.org: adjust commit message; adjust newlines; squash
 two of Kumar's patches into this one.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Signed-off-by: Rob Browning <rlb@defaultvalue.org>