Enhance reuse features to be mixins

[Julien]

It should be possible to inject reusable behaviors in a component.

(defwebcomponent my-component
  [s]
  mixin1 mixin2
  :on-created #(println "created"))

Each mixin is a function that receive a map and produce a modified map. The first one gets the original component map and others each get the previously modified map.