Add attribution property to image-* layouts

[torkleyy]

Is your feature request related to a problem? Please describe.

When using images, it's very common that the license requires attribution. As far as I see, I cannot easily use the image-* layout when attribution is required, so I have to use two-cols.

Describe the solution you'd like

Add a new property to the layout:

---
layout: image-left
image: https://example.com/example-image.jpg
attribution: "Example image" by John Doe, licensed under CC BY 2.0
---

# My slide

Describe alternatives you've considered

I'm not sure about any alternatives, but I'm open to different solutions!