NEW FEATURE REQUEST: Cache filtering by D plus T rating

[+da Bush Man]

Sometimes a little more flexibility in difficulty and terrain filtering would be helpful when looking at a small phone screen that is jammed full of caches.  Let's say you want to target more challenging caches and want to filter out 1.5/1.5 and lower, but you are still interested in a nearby 1.5/4.0 and 3.5/1.0 for example.  If you could set a difficulty plus terrain threshold of D+T = 4, this would filter out the 1.5/1.5 (D+T=3) but not filter out the 1.5/4.0 (D+T=5.5) or 3.5/1.0 (D+T=4.5).  Thanks