Refactor to flag.

[ch3lo]

Hi all,

in my firsts step with Registrator I had some troubles to configure it, because you need a defined order in parameters. Another problem I encountered was it has poor coverage of Unit Test. I made other contributions to this project and here are my other 5 cents.

I propose to integrate a vendor cli to simplify flags and cover the new related code with Unit Test. It's the first step, because I think it could be refactored in another package but I think it's a good approach to include others PR.

What do you think?

[josegonzalez]

I <3 that you added tests, but is there any reason why we couldn't avoid using an alternative flag parsing library in this PR? We can revisit that later, but it seems as though the two bits aren't necessarily intertwined, and could be done separately. If you can, I'd resubmit as two separate PRs, so that we can review each individually.

Sorry it took so long to review this.