Add built-in initializer plugin for external sources

[jaredcwhite]

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

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[jaredcwhite]

I hope to get this into Bridgetown 1.0 but keeping it on ice for the Beta 1 release.