* Change the behavior of the template rendering:
    * Check if the template bare name is present
    * Check if the template is part of a theme
    * Fallback to defaults
* Change the behavior of the shortcode rendering:
    * Call the template rendering function
* Prepend `__zola_builtins/` to most of the default elements in `ZOLA_TERA`
* Add a test to verify the presence and content of a `404.html` page
from a theme's template

Closes #582

Closes #576

This fixes #569 .

`markdown_to_html` is heavily refactored, header-related things is
handled in a second pass.

* add id to continue reading p tag

this commit contains two refactors:
- extract custom link transformations into a function.
- separate some trivial markup generation.

Fix #546

Closes #537

Closes #521

Closes #533

Closes #385