palette-delta-encoding experiment

Transmit palette colors with delta encoding.
Coding gain on scrren_content testset:
overall 0.67%  keyframe 1.37%

Change-Id: I72ce9061dfddf933e9f7530f069955afcb07edf8
diff --git a/configure b/configure
index 742bdde..0a8c3ad 100755
--- a/configure
+++ b/configure
@@ -285,6 +285,7 @@
     bitstream_debug
     alt_intra
     palette
+    palette_delta_encoding
     daala_ec
     rawbits
     ec_smallmul