Stable diffusion support with --format image

[JasonWeill]

Problem

Users cannot run image models using Stable Diffusion using Jupyter AI, because no existing output accommodates images.

Proposed Solution

Add a --format image option for the %%ai magic command.

If the AI model outputs a URL, -f image should cause the output cell to display the image by remotely loading that URL as an inline image, either using markdown ![alt text](image URL) syntax or HTML <img alt="alt text" src="image URL" /> syntax.

If the AI model outputs binary data, and -f image is specified, the output cell should display the binary data returned. If the HTTP call to the model's API specifies a MIME type in its response headers, that MIME type should be used to decode the binary data.

Update the example notebooks and documentation with information about -f image.

Additional context

The option should apply a prompt template (#42) that should tell the AI model to output an image only, without explanatory text.

[JasonWeill]

Use huggingface_hub:stabilityai/model for Stability AI.

We may need to use an image-compatible HuggingFace provider specifically, not LangChain, for image generation. The current HuggingFace provider only supports text output.