Switch argument parsing to `structopt` (which uses `clap`)

[emk]

@Arthaey has suggested a couple of command-line flags that I agree are good ideas (despite my general bias against them). These can be added much more easily if we switch argument parsing to clap. I'll try to take a look at this.

[emk]

Ooh, this is a nice clap wrapper: http://robert.ocallahan.org/2017/11/in-praise-of-rusts-structopt-for.html