Add aria-current option

[johnhunter]

Fixes #14

It adds a useAriaCurrentAttr option so that an anchor matching the activeKey will have an aria-current="page" attribute (see more details here).

Example Nunjucks navigation rendering:

{{ collections.all | eleventyNavigation | eleventyNavigationToHtml({
    activeKey: eleventyNavigation.key,
    useAriaCurrentAttr: true
  }) | safe }}

The PR also includes a refactor to make adding tag attributes a bit simpler.

• [x] Changes have test coverage
• [x] Have verified works in an Eleventy site build
• [ ] Documentation updated