Add flag to enable/disable short-term loudness (S) measurement to decrease minimum file length

[fripso]

Is your feature request related to a problem? Please describe.
This request is related to the following issue: #87 (apologies for necroposting)

ITU-R BS.1770-4 prescribes 400ms (momentary) and 3s (short-term) sliding windows for measurement, and as a result ffmpeg-normalize will disregard files shorter than 3s.

Describe the solution you'd like
The ability to disable/disregard short-term loudness measurements, to enable integrated loudness measurements on short files (<3s).

Pseudo:
ffmpeg-normalize input.wav -o output.wav --integrated-only

Describe alternatives you've considered
The current solution is to pad the file with silence, normalize, then trim, but this is not a scalable solution as it adds multiple passes of processing.

[slhck]

No need to apologize, perhaps the conclusion wasn't so clear. As you can see in #87, this is not an issue related to this tool, but to the respective loudnorm/ebur128 filter in FFmpeg. So it's an upstream bug — or lack of feature, depending on how you interpret it. As long as the filter does not support it, all we can do is print a warning.

I suggest you post this on https://trac.ffmpeg.org/ as a possible enhancement for the loudnorm/ebur128 filter.