feat: add blurry_image Jinja extension

[johnfraney]

Adds blurry_image extension to insert an tag with width & height into a Jinja template. Optionally inserts an image of a specific size.

Usage:

Basic:
{% blurry_image page.image, alt="Image description" %}

With explicit width (must be present in the build folder):
{% blurry_image page.image, 250, id="image-id", class="responsive-image" %}

Example warning output:

Blurring 17 Markdown files from: content
blurry_image: alt attribute missing for image in WebPage.html
Built site in 1.589002 seconds

TODO:

• Add alt, title, and loading attribute support
• Add docs