Adds a copy mode experiment

Experiment to copy motion and mode from block neighbors.

Results:
--------
--enable-experimental --enable-copy-mode:
derflr: +0.727%

With other expts:
--enable-experimental --enable-copy-mode --enable-supertx
--enable-ext-tx --enable-filterintra --enable-tx-skip --enable-tx64x64
derflr: +3.43%

Change-Id: Iae74b9a855a1a690bf76131b42d247bbc54dec17