Lifetime Customer Value for Zen Cart customers

When coach Ryan Waggoner asked me about the lifetime value of my average customer, I was embarrassed to say I didn’t know!  So I added Lifetime Customer Value to the Zen Cart Admin->Customers page in this pull request.  The change is now in the 1.5.6 branch, but it can be easily backported to any 1.5.x version of Zen Cart.  It’s a great way to get visibility into the activity of all you customers, and especially your most valuable ones.