add user controls for image treatments

[Matt]

_data/picture.yml

Jekyll Picture Tag has the following settings fir the theme:

presets:
  default:
    markup: auto
    nomarkdown: false
    formats: [original]
    widths: [414, 600, 700, 800, 1127]
    fallback_width: 700
    fallback_format: original
    data_sizes: true
    dimension_attributes: true
    attributes:
      img:  loading="lazy" data-sizes="auto"'

Most of these settings should remain for markup and performance reasons, but some users may want to use more images sizes, or fewer. Some may also want to use .webp format.

The following settings can be added to the config collection in admin/config.yml

• Make widths options selectable
• Make .webp format optional