Add option to hide index column

Add option to hide index column

The directory size column of the index sorted by date can be hidden
with the new index_fields option in sarg.conf.

This option is added because it was reported that the computing of the
directory size may be very slow. See the feature request:

http://sourceforge.net/tracker/index.php?func=detail&aid=3141177&group_id=68910&atid=522794