setup experimental infrastructure

This patch creates some basic infrastructure for doing bitstream-
incompatible changes to the VP8 encoder. The key parts are:

 - --enable-experimental configure switch, to enable support for this
   incompatible bitstream. This switch is required to be set to enable
   any "experiments"

 - A list for "experiments" which translate into --enable-<experiment>
   options and CONFIG_<experiment> macros.

 - The high bit of the "Version" field is used to indicate that the
   bitstream was produced by an experimental encoder. The decoder will
   fail to decode an experimental bitstream without
   --enable-experimental.

 - A new "vp8x" encoder interface is created to set the experimental
   bit.

 - The vp8x encoder interface is made the default for ivfenc in
   experimental mode.

Change-Id: Idbdd5eae4cec5becf75bb4770837dcd256b2abef