Add "github_url" Jekyll tag for creating "edit this page" links

This commit adds a new {% github_url %} tag plugin. On any page, it
returns an "edit" URL to that page on GitHub.

- If the page is in one of the external sources (as defined in the
  source/_config.yml file), it will link to the source file in that
  project's repo, on the master branch.
- Otherwise, it links to the source file in the puppet-docs repo.
- This requires a monkey-patch to Jekyll! In this commit, that patch is
  applied to our captive copy (see the commit just before this one); in
  Michael Hall's bundler-centric toolchain rework branch, this will be
  done with Git, like everything else around here.
- Keep in mind that edits are immediate if you have push rights to the repo;
  if you don't have push rights, edits result in a fork + pull request.
- The external sources in _config.yml MUST follow a specific format! To recap:
    url: /puppetdb/master (An absolute URL prefix for affected docs, beginning with a slash.)
    repo: git://github.com/puppetlabs/puppetdb.git (A git repo URL, suitable for git clone.)
    commit: origin/master (Any valid Git refspec. Prefer remote branches, for auto-updating.)
    subdirectory: documentation (A directory, relative to the root of the repository, not ending with a slash.)
Add "Edit this page" links to every template
Add url_segment Jekyll tag for treating page.url like an array

Yet again, we get bitten hard by both the paucity of data Jekyll makes
available to page templates and the paucity of logic allowed by Liquid.
I think we're fast approaching a point where we should be considering
other tools.

Anyway, if page.url is /references/3.0.2/type.html, this one lets us say
{% url_segment 1 %} when we want the 3.0.2 part.
Kill the one-trick reference_version tag in favor of the more flexible url_segment tag

This was being used to get PE versions and reference versions.
Exclude edit links from /references URLs

These documents properly live in the Puppet source code, so we don't
want them being edited in situ by helpful readers.