Support alternate output file formats

[drien]

Currently this only works with MP3 output, but should ideally support the intersection of {whatever the API can produce} and {whatever pydub supports}.

• Should parse the format from the output file CLI option
  • If stdout, explicit format arg is required unless we can infer from redirection (os.readlink('/proc/%d/fd/1' % os.getpid()), which is not platform-independent)
• Should probably have enums on the API wrapper class that allow it to raise/present a friendly error message if you try to use an unsupported voice/model/format.