Add support for decoding using FFmpeg in the main example

[shih-liang]

No description provided.

[easyfab]

Could be interesting to have this in main for those you don't want to install ffmpeg.
But you can also simply use directlty ffmpeg to pipe into main.
e.g. : ffmpeg -i file -ar 16000 -ac 2 -c:a pcm_s16le -f wav - | main -m model.bin -f -