Add loading attribute

[nfriedli]

All images shortcodes should have a loading attribute (for example for lazy).

[jmooring]

This is fine, but the images are like 1k...

[nfriedli]

I agree with you. In the vast majority of cases, the images are tiny.

But I think it's a good idea to add this option for 2 reasons:

1. To be consistent with the figure.html shortcode, which features the loading attribute. This way, users really have control over image loading, with the shortcode provided by default.
2. Because I can think of lots of other relevant uses, notably for creating deliberately large QR codes, with lots of text. I've just made a {{< qr level=“high” scale=16 >}}. And as I'll have several on the page, for poetry, it would be useful.

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Personal note. I find the qr.html shortcode very, very well designed.

The variable harvesting part is great for overloading the default configuration. I wanted to create variables in the configuration, but it's so clear that I did it in my local shortcode, for example:

{{- $targetDir := or (.Get “targetDir”) “images/qr” }}
{{- $class := or (.Get “class”) “qr” }}
{{- $loading := or (.Get “loading”) “lazy” }}

Many thanks for your work, @jmooring & @bep.