Reduce intensity levels

##Command Line

NOTE: This doesn't work as expected yet. When 2 intensity levels are selected, this method should generate a binary image with black and white but the way I have reduced intensity levels, the whole image turns dark.

Original image:

###64 levels:

$loomis -command intensity_levels -i testdata/green-bee-eater-grayscale.jpg -levels 64

###128 levels: $loomis -command intensity_levels -i testdata/green-bee-eater-grayscale.jpg -levels 128

##Library Usage

newImage := pkg.ReduceIntensityLevels(img, levelCount)

image is of type image.Image
levelCount is an integer between 2 and 128 (stick to power of 2)