Fixes #667.

Except for the block blocks, if there is an extends, we don't render anything else. However, we're still supposed to generate variables and function calls and so on. This PR fixes it by first generating the template in "extends mode", so we generate variables, make the function calls and everything. And then, we generate the blocks content.

@Kijewski I'd really like to get your input on this PR to ensure I didn't miss something. However please don't push yourself: if you don't have time or/and motivation or/and energy, I'll simply merge it in a week (so on the 27th) and we can always fix potential new issues later on.

To help you reproduce the original jinja implementation, here's the python script:

# You need to install `jinja2`.
from jinja2 import Environment, FileSystemLoader

# The template are stored in the `templates` folder, like `askama` to make it easier for me to test.
env = Environment(loader=FileSystemLoader("templates/"))
template = env.get_template("child.html")
# `value` is a variable sent to the template, not used here but in case you want to...
print(template.render(value=""))