Add a before_render handler callback

[ellmetha]

Description

It would be interesting to provide a new callback allowing to execute code right before a template is rendered when generating an HTTP response in a handler. Such callbacks should be executed in the following situations:

• When the #render method is called.
• When templates are generated as part of handlers making use of the Marten::Handlers::Rendering concern module.

Such callbacks could be used to insert additional data in a template context for example.