Add optional preview pictures/icons to eggs

[lancepioch]

• [ ] Should be an new optional root level parameter in egg configuration
• [ ] Must be a base64 data URI scheme of an image (png) - example below
• [ ] The image must be perfectly square, ideally 100px at least
• [ ] Eggs in this repo must be updated with this change
• [ ] Other eggs must be updated asap afterwards

Valid URI Example

"preview": "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg=="