Add extensions with initial support for pre_render

[jhollinger]

Initial attempt at a formal extension API as part of #341. I've built a POC ActiveRecord automagic preloader against this and #357.

In this proposal, an "extension" is a class inheriting from Blueprinter::Extension and passed to Blueprinter.configure. This PR defines only one extension method: pre_render. It intercepts whatever is passed to .render and returns (potentially) a modified result.

In the simplified version of my POC below, the extension figures out which associations to preload from the Blueprint view (using reflection), and returns the ActiveRecord::Relation with preload called.

Blueprinter.configure do |config|
  config.extensions << MyPreloadExtension.new
end

q = Widget.
  where(...).
  preload_blueprint

# All associations in WidgetBlueprint:default get preloaded
results = WidgetBlueprint.render(q)

class MyPreloadExtension < Blueprinter::Extension
  def pre_render(object, blueprint, view, options)
    return object unless object.is_a?(ActiveRecord::Relation) && object.preload_blueprint?

    preloads = figure_out_preloads(blueprint, view, object.model)
    object.preload(preloads)
  end
end