Added configuration for audio channels

I've added the support for basic audio-channels settings.

As the examples are on the FFMpeg documentation (https://trac.ffmpeg.org/wiki/AudioChannelManipulation), this patch just supports the following scenarios:

* stereo → mono
* mono → stereo (both channels will be identical)
* 5.1 → stereo (first example `-ac 2`)

This is mostly for taking the number of channels down. Taking it up just makes duplicated channels, whereby you have no advantage of.

Does this fix https://github.com/PHP-FFMpeg/PHP-FFMpeg/issues/1?