Group CLI arguments by runner

[josebolos]

Currently pa11y has a single list of CLI arguments, which is the result of pa11y originally supporting only the HTML CodeSniffer runner.

The axe runner doesn't support the same options as the HTML_CS one, due to the way that each one of them works, so a few CLI arguments like --standard are labelled "only used by htmlcs runner".

CLI arguments that are only supported by one specific runner should live in a separate section. This is usually how man pages are structured too for *NIX tools, and it would help people make less mistakes.

[codebru]

I had a look into doing this, starting with looking through the README of Commander.js and sadly there is no supported way with Commander.

One could likely hack it a bit to do a completely custom help output but I feel that would be at a determent to maintainability. Another option would be adding a before or after extra info section defining this information.

I think what would maybe be the most maintainer + user friendly way to do this, is to throw a warning/error when an option is used that is not supported so the user is aware that something they may have intended is not happening? This would likely intersect with #529

I hope this information is useful.