Add option for Adaptive Histogram border pixel handling.

As discussed in #757, this adds an option to change the way border pixels are handled by `equalize_adapthist`.  The old behavior was to leave the pixels not used by the CLAHE algorithm unchanged.  We add the `mode` option to make them 'zero', or to 'crop' the image to the size that was actually equalized, with the default being the current 'ignore' behavior.