Warn when content in extends child

[softdevca]

Top-level content is ignored in templates that extend another. Rinja should warn when this is encountered instead of silently ignoring it to reduce confusion.

This is ignored
{% extends "base.html" %}
This is also ignored
{% block content %}
    <p>Hello, world</p>
{% endblock %}

[Kijewski]

Yes, I think it would be better if we make this a syntax error! But I guess is would be a breaking change, because it's a documented feature that any data outside of {% block %} is ignored.