Feature: setting the required silence length

It would be great to be able to set the required silence/pause length after a sentence programmatically.

I reduced the `min-trailing-silence` rules in the `model.conf` and it feels great typing keys with Numen, but it also affects the literal transcription which uses the same model, and I don't want to load two copies of it (or mess with the models really).

Perhaps the API could be:

rec.SetSilence1(0.123)
rec.SetSilence2(0.123)
rec.SetSilence3(0.123)
rec.SetSilence4(0.123)

for Kaldi's rule1/2/3/4. But just being able to set `rule2` would be very nice.

Related issues: https://github.com/alphacep/vosk-api/issues/1329 https://github.com/alphacep/vosk-api/issues/380