Add color conversion alias

[wuhaochen]

Adding some color conversion code alias same as https://github.com/opencv/opencv/blob/850be1e0874a4881b94392773d2f1702344658ac/modules/imgproc/include/opencv2/imgproc.hpp#L536

This change helps with code readability especially for demosaicing.

I left out other alias in the hpp file for now as they seem to be addressing only different naming in YUV format. The only caveat to this change may be that the String function for ColorConversionCode in https://github.com/hybridgroup/gocv/blob/e2db8f81cfce48d9b2c2d4ce92219a7a4a021bfe/imgproc_colorcodes_string.go can return a different name for a constant, but it should be an ok compromise.