Add -o/--output flag to write transcription text to a file (#113)

* add -o/--output flag to write transcription text to a file

When -o <path> is given, the CLI writes only the transcribed text (no
metadata, no timestamps, no timings) to that file. Stdout is unchanged.
In batch mode each utterance's text goes on its own line.

 examples/cli/main.cpp | 33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

* tweaks

---------

Co-authored-by: CJ Pais <cj@cjpais.com>