Add previous option

[ChrischaoWang]

No description provided.

[grero]

You should be able to do something like what I did here #29 to update the previous options. It' a bit convoluted, but because the setplotopts function needs to traverse each path of the pop-menu, I couldn't find a better way to do it.Basically, the _opts variable is a dictionary that holds the name and the value of the variable that is being updated from the popup menu.