Heatmap: Add option to invert colors to help find low value items

[dirtboggle]

It would be nice to be able to highlight least valuable items so that worthless bank items can be identified and cleared out. Currently, high value items (millions) are too high and cause colors to concentrate across huge value differences in value. For example, a bank with its highest value items consisting of zenyte jewelry will have those items highlighted red, and nearly everything else blue. In this case, it would be impossible to distinguish the difference in blue between item stacks worth 10 gp and 100k gp.