lavf/matroskaenc.c: add early support for colour elements

lavf/matroskaenc.c: add early support for colour elements

Adding early support for a subset of the proposed colour elements
according to the latest version of spec:
https://mailarchive.ietf.org/arch/search/?email_list=cellar&gbt=1&index=hIKLhMdgTMTEwUTeA4ct38h0tmE

Like matroskadec, I've left out elements for pix_fmt related things
 as there still seems to be some discussion around these.

The new elements are exposed under strict experimental mode.

Signed-off-by: Neil Birkbeck <neil.birkbeck@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>