Implement contrast-limited adaptive histogram equalization (CLAHE)

[ssnover]

I'll be poking at implementing CLAHE for this crate over the next month. Should be about equivalent to this MATLAB function: https://www.mathworks.com/help/images/ref/adapthisteq.html and this OpenCV function: https://docs.opencv.org/3.4/d6/db6/classcv_1_1CLAHE.html.

[theotherphil]

Looks like a very useful addition, thanks!