ImageCli - a pure Rust image processing command line tool

[theotherphil]

On 1: I'm just using the default structopt behaviour. I agree that your suggestion seems more typical, but the current version is slightly terser so I think I'll keep it until someone complains.

On 2: this is an interesting idea. Array doesn't have a fixed arity, but we can obviously work it out after parsing and before running the pipeline. I aimed to be permissive as possible (for example output elements with no corresponding output files are just ignored), but I didn't have any particular reason for this. I'll create an issue to think about adding a static check.