rules option ignored about pa11y HOT 1 OPEN

Hi @edsnowden, thanks for this report, and for your code and logs which made it straightforward to understand the issue and reproduce the behaviour. Also, that is a very pleasant amount of memory.

For HTML_CodeSniffer: I'm reasonably confident this is the currently intended behaviour of options.rules, going by the name of its non-obvious CLI counterpart, --add-rule, and by this test's name. I agree that rules is not a very descriptive name though, if I'm understanding this correctly, so maybe we should improve how we explain their equivalence.

For axe: --add-rule and options.rules are currently documented as being only for HTML_CodeSniffer, but I think we use this property internally for a different purpose for axe, where the intended behaviour appears to be 'set' rather than 'add', according to the name of the test I just linked, but I'm actually seeing the 'add' behaviour locally for axe too.

Are you comfortable with this behaviour, with the understanding it represents addition rather than replacement? Some other changes we could make:

1. (breaking) simply rename rules to be clearer; probably not a good idea since it would inconvenience current users of the property
2. (minor) same as (1), but also leave rules (now an alias for that new property) in place
3. (minor) explain this better in our documentation, whether in code or markdown. When you first learned about options.rules, where were you looking? We could try to clarify this to readers there

from pa11y.