Allow combining 2 mono streams to analyse them as stereo signal

[mbirth]

We are using your scanner with MXF files which consist of 1 video stream and 8 mono audio streams. However, these 8 streams are actually 4 stereo channels with separate streams for L/R audio.

The scan tool only analyses the first audio stream it can find, which is the left channel audio in our case. So all LUFS values are 3 LU short (for even loudness distribution between the stereo channels).

It would be awesome if you could add a flag to make the scanner analyse the first 2 audio tracks as stereo, if the first track is mono and there's at least a second one.