✨ feat: add invertable image shortcode

Useful to invert diagrams, graphs, etc. in dark theme.
To use it in a markdown file, add a line like:
{{ invertable_image(src="/img.png", alt="alt text") }}

templates/shortcodes/invertable_image.html

@@ -0,0 +1 @@
+<img class="invertable-image" {% if src %}src="{{ src }}"{% endif %} {% if alt %}alt="{{ alt }}"{% endif %}/>