Feature: Make audio_norm.sh test to see if the file is video or audio

[iamdamosuzuki]

It should be possible for audio_norm.sh to process audio or video files without breaking

[eddycolloton]

I have the audio_norm.sh script working locally with a few test files, and I've added some PRs to show the small changes ( #13 #14 #15 ). The ffmpeg command works "as is" for both audio and video, but I've added an if statement for a ffmpeg command that omits the -c:v flag if the format isn't video, as proof of concept.