Allow configuring Axe locale

[goulvench]

Hi, I've searched in the codebase for locale and lang(uage) but nothing relevant comes up.

Requirements

• Add a report_locale method
• Pass the selected locale to Axe-core (axe.configure({locale: ?}) before running checks.

User Story

We're automatically auditing a website to make sure that code changes don't introduce accessibility issues.
The website maintainers are not native English speakers, and would be better able to understand and fix issues using the translated messages.
Axe-core includes a dozen locales, it would be great if the issues reported back to Rspec could be translated.

Acceptance Criteria

expect(page).to be_axe_clean.report_locale(:ja) returns japanese messages.