Fix duplicate links not being preloaded

[daun]

• Index visible links by href first, by element second
• Fixes issue No prefetching occurs for duplicate links #387 where duplicate links with identical hrefs won't preload at all if one link is visible and another one isn't

Closes #387

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[XhmikosR]

@hadyan could you review this PR please? :)

[XhmikosR]

@addyosmani please review this PR too