Add irqtop command

Add new "irqtop" command (contributed par Robert Elliott from HP).
This command monitors differences in /proc/interrupts and
/proc/softirqs per CPU, along with CPU statistics.
irqtop is currently a Perl script that you can find in the contrib
directory.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>