Add column for average ping delta

[Robert Sander]

It would be nice to have a column that displays the delta between the average ping time of two "stations" on the route.

This is useful to see where the ping time increases (e.g. crossing an ocean).

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: mtr 0.80-1ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-32.51-generic-pae 3.2.30
Uname: Linux 3.2.0-32-generic-pae i686
ApportVersion: 2.0.1-0ubuntu8
Architecture: i386
Date: Sat Oct 13 21:10:41 2012
ProcEnviron:
 LANGUAGE=de_DE:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: mtr
UpgradeStatus: No upgrade log present (probably fresh install)

[rew (r-e-wolff)]

Wouldn't be too hard to add.

Go ahead and provide a patch! :-)