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.

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

[johnhunter]

@zachleat Is there anything required for this PR to be merged? I'll raise a PR on the docs site once this one is accepted. Thanks.

[earlman]

+1 for this! 🙂

[Dexus]

@zachleat @binyamin Please!

[binyamin]

@Dexus I don't have write access to this repository. Sorry.

[johnhunter]

@zachleat is there anyone who could review the PR even if they can't merge it? It might help move things along.

Thanks.