* add shortcode 'invocation' variable to allow a shortcode to track how many times it has been invoked in a given Markdown file
* use closure (implicit struct) instead of explicit struct for invocation tracking
* update variable name to "nth"
* Change zola serve to load HTML from memory instead of disk
* Be smart about assets copying
* Be a tiny bit smarter on template changes
* Add zola serve --fast
* Replace hack for newline support in shortcodes with new hack
* Be a bit more space efficient/accurate with naming
* Boil newline/whitespace shortcode test down to the essentials
* Make sure the new \n and \s chars in old tests are properly represented
* Support markdown templates and shortcodes
* Refactoring .md/.html shortcode behaviour
* Add test for markdown shortcodes
* Add an html output test for markdown based shortcodes
* Add documentation for Markdown based shortcodes
* 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
