Add silent audio to videos with no audio

[Giovanni Musto]

Apparently my Samsung TV does not like audioless videos, and says "Unsupported audio codec". Maybe we can add an option to add empty audio to such videos. A solution is already found here: https://stackoverflow.com/questions/12368151/adding-silent-audio-to-mov-in-ffmpeg

We could add "-f lavfi -i anullsrc -shortest" after the video input and then use "-map 1:a" (1 because it's another stream, could be higher) to output them, removing the "-an" switch.