Warn before overwriting output file

I forgot how opusenc worked and tried to encode two recordings via `opusenc one.wav two.wav` thinking it would encode `one.opus` and `two.opus`, and instead overwrote and lost the second one. 😞 

The Opus tools could, like the FLAC command-line encoder for instance, not overwrite the output file unless a `-f` or `--force` option is given.