Initial work towards #819, #884
Fixes #704

This PR expands on the Source Manifest functionality so that multiple per-collection content directories can be specified. So you can load content into pages and/or other collections as you wish.

In addition, a new external_sources initializer plugin is provided so a site owner can easily add their own source manifest in order to load external content. Here's an example config/initializers.rb file:

init :external_sources do
  contents do
    docs File.expand_path("../../test_content/pages", __dir__)
  end
end

collections do
  docs { output true }
end

config.defaults << {
  scope: { collection: "docs" },
  values: { layout: "page" },
}

TODO:

• Add watcher for live reload
  note: sorta works with additional_watch_paths << File.expand_path("../../test_content/pages", __dir__) …just gotta make this automatic
• Tests
• Documentation