Dig into CLI option parsing more

[jpeddicord]

I went with clap originally as it seemed nice, and it's worked out well so far. But I didn't put a lot of thought into it. Actions:

• Investigate whether using it in yaml mode adds significant bloat/start-up cost
• Figure out what appropriate command line options should be; KISS
• Look at other libraries too; structopt (wraps clap) was recommended to me