Allow to open the diff in an external viewer

[RandallFlagg]

Is your feature request related to a problem? Please describe. I would like to see the difference with an external viewer(e.g. beyond comapre)

Describe the solution you'd like A configuration that will let me specify the path to the viewer and arguments if needed

Describe alternatives you've considered It is not an alternative but I would like it to work the same way it works in tortoisegit

Additional context Add any other context or screenshots about the feature request here.