feat: allow extra_head and custom content additions

[mladedav]

I want to inject custom script tags into head and body. For head, I have added a custom tag like hyde uses. Content can be overriden in base and it will be appended at the end of whatever extends base.

This allows users to override base.html like this:

{% extends "zolanight/templates/base.html" %}

{% block extra_head %}
<style>
  {# some css #}
</style>
{% endblock extra_head %}

{% block content %}
  {# some javascript #}
{% endblock content %}

[mladedav]

It might be better to have another extra_body block instead of piggy-backing on content so that the script is guaranteed to be at the end of the page. I'm not sure though and did it this way to not introduce new blocks if they're not needed.