Support local images

[cloverich]

Support local images. That is, image urls that point to local files. This is part one of at least a two part image support series and separates the harder decision of: Will Chronicles support native image upload and serving, or delegate that to some third party service (more likely). By implementing local images first, I do many parts of the same things needed for remote image support.

• Display images when saved documents are loaded
• Convert text to image when text is pasted in
• Convert text to image when image text is typed on its own line, after hitting enter
• Consider allowing label editing

Deferred:

• Image embed toolbar
• Drag and drop image
• Move image block once created
• Proper security handling (hence local image support)

[cloverich]

image label editing added thanks to plate and 2ed6a5e