fix: Adds handling of webmanifest.

[css-optivoy]

This adds handling of webmanifest links like:

<link href=./manifest.webmanifest rel=manifest>

Context:

• Web Manifests
• The application/manifest+json MIME type

This PR fixes #475

[css-optivoy]

Aside: It's possible to inline manifests like:

<link rel=manifest href="data:application/manifest+json,{ ... }">

But this PR does not attempt to implement this new scheme, as I think that is better left as a follow-up PR if desired.

Source: https://gist.github.com/marcoscaceres/7783977