Add ability to normalize audio

[brarcher]

Normalizing audio requires two passes, one to determine the peak audio value and one to perform the necessary filter.

https://peterforgacs.github.io/2018/05/20/Audio-normalization-with-ffmpeg/

[novadeviator]

there are two ways:

Loudnorm filter

http://ffmpeg.org/ffmpeg-all.html#loudnorm (single and two-pass)

dynaudionorm filter

http://ffmpeg.org/ffmpeg-all.html#dynaudnorm (seems like single pass?)

that would be really beneficial if it would be included in the near future!