aXe runner support

[cfq]

Pa11y uses HTML_CodeSniffer by default but unfortunately HTML_CodeSniffer isn't very good when it comes to finding barriers that can be detected by automated accessibility checkers, and aXe is quite a lot better.

Pa11y has support for different runners and it would be great we could specify which one we prefer.

[ehmicky]

Hi @cfq,

This makes sense. Would you be interested in sending a PR to add support for this?
Thanks!

[cfq]

Thanks @ehmicky, I definitely would!

[ehmicky]

Thanks! Please let me know if you need any help 🙏

[swyxio]

just for some historical context - this plugin originally used axe, and we could not get it to work in netlify's buildbot for whatever reason: #2

hence I pivoted to use pa11y AFTER spending 1 whole week failing at getting axe cli to work.

good luck, i hope you figure it out.