#4 Add getAudioStreamId and getVideoStreamId methods

[Jonathan Owens]

When using ffmpeg-php output to generate command-line encoder strings, it is useful to be able to read the stream IDs of the video and audio streams in a file. This patch adds the getVideoStreamId() and getAudioStreamId() methods to read these IDs.

[Jonathan Owens]

get stream ID patch

[Todd Kirby]

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applied, thanks for the patch!