Make Rendering context pluggable

Depending on the use cases, we may want additional stuffs to be available in the rendering context :

• audit entries related to the document
• some getParent getChildren function
• some pure helper functions (like access to vocabularies or date time formatting)
• access relations ...

So rather than trying to have a comprehensive static context, we should make the context plugable so that any Nuxeo plugin can contribute some content/code in the context via a simple factory.