This bit appears in the sequence header. At the moment, it does
nothing, but it will mean that we don't encode (or read) chroma
planes.

This patch adds the flag to the sequence header, and also adds an
encoder option (--monochrome) which enables it. At the moment, this
doesn't do anything except cause the bit to be set correctly in the
header.

Change-Id: If5598412c1c75101553d7f8f9098f9ed1163514e