Closes #20. Add installed callback for Middleware classes.

Middleware classes can now define an `on_install` block to be called when an instance of it is created and added to the middleware stack of an agency. The callback will receive the agency as the only argument, and will be executed in the context of the Middleware class (not the instance. This is not set in stone, comments are welcome).

Since #20 was opened, the `Schemable` module was implemented, changing the way attributes are added to objects. Because of this, and because of slightly different opinions on implementation, the actual code to achieve the same result as the example given in the issue (adding a `neighborhood` attribute to `Shark::Vehicle`) would simply be

```
class NeighborhoodMiddleware < Shark::Middleware
  on_install do |agency|
    Shark::Vehicle.attribute :neighborhood, type: String, nilable: true
  end
end
```

agency.rb

@@ -65,7 +65,10 @@ def create_managers
     def create_middlewares
       instance_list = []
       @middleware = configuration.middlewares.inject(nil) do |app, (klass, args, kwargs, config)|
-        klass.new(app, *args, **kwargs, &config).tap{ |inst| instance_list << inst }
+        klass.new(app, *args, **kwargs, &config).tap do |inst|
+          instance_list << inst
+          klass.installed(self)
+        end
       end
       # Some Middlewares will use background threads to process work. By
       # sleeping for a short time between checks, those threads can work

middleware.rb

@@ -40,6 +40,17 @@ def self.register_handler namespace, event, &handler
     end
 
 
+    # A callback to be run when a Middleware instance of this type is
+    # installed in an Agency.
+    def self.on_install &block
+      @installed_callback = block
+    end
+
+    def self.installed agency
+      @installed_callback&.call(agency)
+    end
+
+
     attr_accessor :app, :storage