add flag to make_rgb_image() to encode nans in transparency layer

Add boolean option "make_nans_transparent" to make_rgb_image(). If set and output is png, will add alpha layer to output file, setting any nans in the input data arrays to transparent. A pixel containing a nan in any color will be set to transparent... this makes sense to me as the default behavior, although I could add an additional option that requires a nan in all colors for transparency.

This follows from issue https://github.com/aplpy/aplpy/issues/137, wherein set_nan_color() has no effect for RGB images. set_nan_color() still has no effect, but this allows a workaround.