Add StereoMode parsing since this parameter is supported for 3D WebM

Change-Id: I2f0210a42b1218433e67b841dbc65e37d0137112

ebml_header.py

diff --git a/ebml_header.py b/ebml_header.py
index de21ce2..ddfa5fa 100644
--- a/ebml_header.py
+++ b/ebml_header.py
@@ -272,6 +272,7 @@
      DISPLAY_WIDTH : ('Display Width', LEVEL_4, EBML_UINTEGER),
      DISPLAY_HEIGHT : ('Display Height', LEVEL_4, EBML_UINTEGER),
      DISPLAY_UNIT : ('Display Unit', LEVEL_4, EBML_UINTEGER),
+     STEREO_MODE : ('Stereo Mode', LEVEL_4, EBML_UINTEGER),
      ASPECT_RATIO_TYPE : ('Aspect Ratio Type', LEVEL_4, EBML_UINTEGER),
      FRAME_RATE : ('Frame Rate', LEVEL_4, EBML_FLOAT),
      AUDIO : ('Audio', LEVEL_3, EBML_SUB_ELEMENT),