Allow selecting required API version via --api-version

[imp]

I have quite a lot of positive experience with srtuctopt, and clap 3 is essentially it, so I rewrote arg parsing to use Parser trait. Hope you don't mind.

Selecting required version - please have a look, I believe I got it right but would love to get feedback.

And the last one - macro_use is so Rust 2015, just couldn't help it :)

[clux]

Hey, thanks for this! I think either clap / structopt makes sense. Is there any particular benefit you see from structopt over clap?

I did start out with clap 3 beta because I wanted dynamic auto-complete of arg1 via #6. If structopt uses clap 2 under the hood, I don't think it will support it, but maybe I am wrong.

I see you've also started doing bits from #3 in here, appreciate it!

[imp]

To the best of my knowledge clap 3 is essentially clap 2 + structopt. One visible advantage is that no *-beta.x versions appear in dependencies. Other than that should be identical. I don't believe there is any functionality gap,- I am fine with both.

As for generating autocompletion - the autocompletion code runs out of the kopium context AFAIK. That said it should perfectly ok to use kopium to provide required completion data for the autocomplete scripts.
I.e. possible crd names may be fed by kopium list-crd in order to enable auto-completion. The alternative is to use kubectl, however that creates unneeded dependency.

Also - I see you prefer --api-version to --version, which makes sense.

[clux]

Ok, I had a look at clap's generation and it's still not great. The dynamic completion support is still open: clap-rs/clap#1232 so no need to use a clap beta for now. Your changes with structopt simplifies the generation so let's go with this.

I don't have anything to really nit at in this PR, so happy to merge unless you want to do more of #3 here while you are at it.

[imp]

Alright, so structopt it is, squashed and rebased.

[clux]

Nice. Merging this one while it's ready. Thank you.