refactor with getopts in order to make some parameters optional

[windowsrefund]

Hello Phil,

Thank you for providing this script. I found it very useful but I wanted to output something with no artist name and/or title. With those goals in mind, I decided to refactor things to use getopts. I also added a fair amount of error checking along with sane defaults where useful. Also added the ability to specify a output file name (-o) when the -a (artist name) and -t (title) are omitted.

If you're at all interested extending this script beyond the current use case, it'd be great to see about changing various colors and/or ffmpeg filters. I'm a newb when it comes to ffmpeg but think it'd be fun to perhaps incorporate support for showspectrum