Added monochrome option to the decoder.

When this is set (use --monochrome), all decoded frames
will be given constant chroma planes.

If the rawvideo option is used in conjunction with the
monochrome option (i.e. --monochrome --rawvideo), the
written output will only consist of the Y (luma) plane.

Change-Id: I967817f1c3ebb1162fa9771b51cf6431120b835c