Add Demo:Histogram Equalization

docs/src/demos/histogram_equalization.md

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+# [Histogram Equalization](@id histogram_equalization)
+
+This illustration enhances an image with low contrast, using
+a method called histogram equalization. Observe that image (2)
+has roughly linear cumulative distribution function.
+
+Histogram equalisation was initially conceived to  improve the contrast in a
+single-channel grayscale image. The method transforms the
+distribution of the intensities in an image so that they are as uniform as
+possible [1]. The natural justification for uniformity
+is that the image has better contrast  if the intensity levels of an image span
+a wide range on the intensity scale. As it turns out, the necessary
+transformation is a mapping based on the cumulative histogram.
+
+```julia
+using Images, ImageView, Colors, TestImages, Gtk.ShortNames
+
+img = testimage("moonsurface")
+imgeq = histeq(img, 256)
+imgadap = clahe(img, 256)
+
+grid, frames, canvases = canvasgrid((1, 3)) 
+
+imshow(canvases[1,1], img)
+imshow(canvases[1,2], imgeq)
+imshow(canvases[1,3], imgadap)
+
+win = Window(grid)
+Gtk.showall(win)
+```
+Here's the result in IJulia:
+
+![moonsurface](../assets/demos/histogram_equalization.png)