MP4 input files support

[dacacioa]

Nowadays appu only allows mp3 input format files. We could change appu to accept it.

[ifosch]

I've added this to be added in a future release. I'll write a README.md with a section detailing the stages, milestones and roadmap.

[ifosch]

It could be interesting to review the mp4 handling operation from the README.md (still pending to be published at this time). These will include instruction to manually extract audio from video files using ffmpeg, but also how to contact a two-parts recording. This could be interesting to build in one or more separate tools.