fix: parse captions from tracks with trackId of zero (#247)

While this goes against the spec, there's broken content out there and it's easy to add with minimal maintenance burden.

lib/mp4/caption-parser.js

@@ -231,7 +231,8 @@ var parseCaptionNals = function(segment, videoTrackId) {
 var parseEmbeddedCaptions = function(segment, trackId, timescale) {
   var seiNals;
 
-  if (!trackId) {
+  // the ISO-BMFF spec says that trackId can't be zero, but there's some broken content out there
+  if (trackId === null) {
     return null;
   }
 
@@ -332,8 +333,9 @@ var CaptionParser = function() {
       timescale = timescales[trackId];
 
     // If an init segment has not been seen yet, hold onto segment
-    // data until we have one
-    } else if (!trackId || !timescale) {
+    // data until we have one.
+    // the ISO-BMFF spec says that trackId can't be zero, but there's some broken content out there
+    } else if (trackId === null || !timescale) {
       segmentCache.push(segment);
       return null;
     }