[Super Editor] - Markdown: Parse back to back images with captions

[matthew-carroll]

[Super Editor] - Markdown: Parse back to back images with captions

Example, handles:

Document with image with caption.
        
A caption:
![Image 1](https://images.com/some/image1.png)
B caption:
![Image 2](https://images.com/some/image2.png)
C caption:
![Image 3](https://images.com/some/image3.png)

Paragraph after the captioned image.