Generate `names.json` for re-use

[KaddaOK]

If a names.json isn't specified or autodetected, TASMAS will interrogate the user to confirm what each speaker's name should be in the transcript.
It would be smart if it then offered to write the values it just confirmed with you into a names.json so you wouldn't have to provide them again if you re-processed, or for the next time you run with the same speaker names.