Allow transclusion of API endpoint results

[jmatsushita]

This could be an underlying facility for #40 or #41 where the source of a page could describe an API endpoint (ideally serving a JSON object) and some form of templating to display data.

For instance:

• have a /issues/index.md page with:

---
source: https://github.com/api/blahblah
layout: issues

---

• have a /layouts/issues.jade file such as

each issue in issues
  li= issue.name
    a(href=issue.link) issue.description

This should be fairly easy with Metalsmith and the metadata plugin after an API fetch to file step, or by creating a plugin that directly consumes the API at each generation run (which could be based on the contentful or primsic metalsmith plugins which do this).

• then the issue list could be transcluded from a page with :[](/issues)
• then an individual issue could be transcluded with the awesome syntax :[](/issues#2) (related to Transclusion of blocks via fragment identifiers #32 for transclusion of fragment blocks)