Allow aggregation of full origin URL JS/CSS sources

[zachleat]

Starting in 0.2.0 <script src="filename.js"> and <link rel="stylesheet" href="filename.css"> contents are aggregated under the normal compilation APIs.

However, using a full URL in these sources will throw an error (use webc:keep to leave these as is).

This enhancement request is to add support for <script src="http://example.com/filename.js"> and <link rel="stylesheet" href="http://example.com/filename.css">, probably using @11ty/eleventy-fetch.