Add NV12 support

With unit test verifying PSNR

Add new command line option --nv12 to aomenc to accept NV12 input.

Bug: aomedia:3232
Change-Id: Id70ea826f4b53901dea14abb7e719e1efffa1de4
diff --git a/apps/aomenc.h b/apps/aomenc.h
index d717b24..935d5fc 100644
--- a/apps/aomenc.h
+++ b/apps/aomenc.h
@@ -24,6 +24,7 @@
   I422,  // 4:2:2 8+ bit-depth
   I444,  // 4:4:4 8+ bit-depth
   YV12,  // 4:2:0 with uv flipped, only 8-bit depth
+  NV12,  // 4:2:0 with uv interleaved, only 8-bit depth
 } ColorInputType;
 
 /* Configuration elements common to all streams. */