Add option to provide loudness information by way of a loudness box ('ludt')

[gordon]

HLS calls for a loudness box:

2.19. In fMP4 files, you SHOULD provide loudness information by way of a loudness box ('ludt'). When present, the loudness box takes precedence over any loudness information in the audio stream.

Ref ​https://developer.apple.com/documentation/http_live_streaming/hls_authoring_specification_for_apple_devices

FFmpeg can be used to normalize and provide this data (​https://github.com/slhck/ffmpeg-normalize)

It would be helpful if loudness metadata could be provided on the command line and then written to the 'ludt' loudness box when using the dash and hls output formats