Process page contents with Jinja

[dreikanter]

Transform this

title: Some page
template: page

Page content.

into this

{% extends "page.html" %}
{% block content %}
Page content.
{% endblock %}

and use env().from_string() to get the template (already populated with page content).

This change will enable Jinja2 syntax inside page content, which allows to use it to insert images this way:

{{ img(451) }}

On template extension: http://jinja.pocoo.org/docs/templates/#template-inheritance