Add build callbacks to Builder plugins and allow arbitrary instantiation

[jaredcwhite]

Resolves #421

This also changes the behavior of the resource_data_for_id method builders can provide — before it was a class method, which meant no access to instance methods or variables. Now it can be an instance method. For example:

class ResourceData < SiteBuilder
  def resource_data_for_id(id)
    { title: "Here's data!" } if id == "builder://ResourceData/my_page.html"
  end
end

[render]

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[render]

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