Added support for OpenColorIO displays with the new --ociodisplay flag.

The new --ociodisplay flag applies color transforms from an
OCIO display/view combination. Optional keywords are "from" (source
colorspace), "looks" (optional looks overrides, supports empty string
to disable looks), and the same "key" and "value" keywords as --ociolook.

I think it would be useful to be able to request the default display and view,
but I'm not sure what the syntax should be. The best I could come up with
was using empty strings to indicate defaults:

    --ociodisplay "" ""              # default display and view
    --ociodisplay "sRGB" ""          # default view for the display named "sRGB"
    --ociodisplay "sRGB" "FilmLook"  # explicitly specified display and view

I'm not sure that's good syntax, so it's commented out at the moment. Feedback
is appreciated!