smart thresholding should be a class

[kylemcdonald]

you regularly want to threshold using something like color, brightness, maybe HSB, or RGB. this is inside ContourFinder right now, but you may want to use it separately.

[micuat]

Hi @kylemcdonald, also it's better to have int thresholdType = invert ? THRESH_TOZERO_INV : THRESH_TOZERO; options, which preserve original image levels instead of binarization, for https://github.com/kylemcdonald/ofxCv/blob/master/libs/ofxCv/include/ofxCv/Wrappers.h#L121