Allow printing the date in compact ISO 8601 format. E.g.:

date    :	Wed Nov 11 14:55:04 MST 2009
date -i :	20091111T145504


The 8601 format allows rc scripts to create compact timestamps for log
entries, is easier to manipulate with sed/grep/etc., and is a natural
key for sort(1). ('date -n' is compact and sortable, but isn't human
readable or regexp friendly.)