Expose a function that can be used in templates

[mrksbnch]

As far as I can tell, this plugin currently only adds the srcset and sizes attributes to images that are used in kirbytext. What about the other images in a page, e.g. those that are included in a template?

headerimg:
    label: Image Label
    type: image
    required: true

<img src="<?php echo $page->image($page->headerimg())->url() ?>" alt="...">