* Change continue-reading to use a span element instead of named anchor

* Fix all tests

The issue with the check_site test hanging and timing out seems to
be related to a similar reqwest issue, which was ultimately due to
an upstream bug in tokio and may be fixed in tokio 0.2.7 onward.

"[…] `&` normally indicates the start of a character entity reference or
numeric character reference; writing it as `&` […] allows `&` to be
included in the content of an element or in the value of an attribute."

From: https://en.wikipedia.org/wiki/HTML#Character_and_entity_references

* cargo fmt & clippy

* Skip anchor checking for URL with prefix in config

* Allow ignored_content to support markdown files

* Add test for markdown supported ignored_content

Part fix of #742

Closes #607

Fix #604

* 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 #521

Closes #533

To override all child pages

Closes #397
Tests from PR #434

Closes #413
Closes #408

Aliases that have no directory nesting sneakily avoid the code path that ensures the directories exist

Closes #485

Close #446
Close #260
Close #478
Close #284
Close #480

Fix #205

This allow to use base_url in robots.txt, to reference a sitemap for
example.

Closes #443

Closes #403