Add read/write support for track disposition information

docs/guide/reading-media-files.md

@@ -106,6 +106,10 @@ track.languageCode; // => string
 
 // A user-defined name for this track.
 track.name; // => string
+
+// Information about the intended usage of the track
+// (default, commentary, hearing-impaired, visually-impaired, etc.)
+track.disposition; // TrackDisposition
 ```
 
 #### Codec information

docs/guide/writing-media-files.md

@@ -56,6 +56,7 @@ output.addVideoTrack(videoSource, {
 output.addAudioTrack(audioSourceEng, {
 	language: 'eng', // ISO 639-2/T language code
 	name: 'Developer Commentary', // Sets a user-defined track name
+	disposition: { commentary: true }, // Sets additional flags in the file
 });
 output.addAudioTrack(audioSourceGer, {
 	language: 'ger',