Reverse upgradeable portals

[rongou]

First off, I'd like to say the upgradeable portals plugin works great! It's exactly what I've been looking for.

A feature request: is it possible to add a layer to show reverse upgradeable portals? For example, if I'm level 8, show all the portals that can be upgraded by the other faction to level 7 or 8. This way after I take down all the L7/8's in the area, I can focus on those. :)

Thanks!