Add option to skip encoding

[cceyda]

Added option to skip encoding by passing re_encode=False. this also bypasses ffmpeg requirement if the files are already in this format. Use case: I already had files in .wav format with -ar 16000 -ac 1. This saved me an extra read+encode+write. It is possible people working with audio data to do ML stuff also has their data in this format.

Added option to skip existing to script. (using the existing skipifexist)