Allow dash in SequentialJsonlWriter

[desh2608]

This makes piping possible for several CLI binaries such as trim-to-supervisions etc. An example piped workflow for combining LibriSpeech train set, cutting into shorter segments, and shuffling:

lhotse combine data/manifests/librispeech_cuts_train* - |\
  lhotse cut trim-to-alignments --type word --max-pause 0.2 - - |\
  shuf | gzip -c > data/manifests/librispeech_cuts_train_trimmed.jsonl.gz

[pzelasko]

And so we have went full cycle bash -> python -> bash 😄

Thanks LGTM!