select first appropriate stream index by default

[agrathwohl]

Closes #6

Please test out this new approach to AudioTrack.from. Once we are looking good I will merge & cut a new release.

Note: This is not a breaking change; the original way of AudioTrack.from(source, { streamIndex: 0}) should still work as expected. in order to select the correct streamIndex automatically (without specifying opts.streamIndex), you need to give the results of source.probe as an opts argument in AudioTrack.from(). See the following example:

const src = new Source('http://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4')

src.probe((e, probe) => {
  const audioTrack = AudioTrack.from(src, { probe })
  console.log(audioTrack.properties.streamIndex) // 0
})

This particular source video file is unique in that it puts the audio track in index 0, while the video track lives in index 1. This is the opposite of what the standard suggests, so very few videos are authored in this manner. This makes it a good testing grounds. Even though AudioTrack.DEFAULT_STREAM_INDEX == 1, it still chooses the correct index.

[agrathwohl]

@r-token I updated the issue text to clarify that this is not a breaking change. It's an improvement intended to make it more convenient to select the correct track without having to know its streamIndex first.