Add: Support CMYK mode with 5 channels, seems no work.

I try to support CMYK mode with 5 channels(CMYK and alpha channel), but it seems no work. Here is what I refer to: https://github.com/alco/psdump/blob/master/libpsd-0.9/src/image_data.c , function static psd_status psd_combine_cmyk16_channel(psd_context * context). Hope you can fix it.