HTML image output has no default size limit

[wheelercj]

On websites, image sizes are usually defined separately from image HTML. Since styles in separate files are ignored · Issue #121 · Stardown-app/Stardown, the image size limits are lost in Stardown's output when the output is set to include at least some HTML. This can result in each image taking up the entire screen.

Allowing users to choose a maximum height and maximum width for images would not make the output perfect, but it would be a big improvement.