Increased m4a and mp3 quality and optimised search for ffmpeg

m4a has 2 options now-fast and quality
fast downloads in lossy AAC format as a mp4 and renames to m4a(seems stupid but it works trust me)
quality downloads in lossless opus format as a webm and converts to m4a thus retaining quality
mp3 also now downloads in opus format and converts to mp3

program now uses one of the functions from the pydub library to check for ffmpeg