feat: Add preview images for embeds

[antheas]

Allows for specifying in markdown the description, image, and alpha values.

Accepted formats:

# Simple
# Uses a default blend setting with purple and crops
preview: ../img/distrobox.png
description: |
   Distrobox is ...

# Expanded
preview:
   src: ../img/distrobox.png
   alpha: 150 # from 0 (invisible) to 255 (fully visible)
   contain: True # For images with transparent sides only, center the image without cropping
description: |
   Distrobox is ...