Add support for image upload to multimodal models

[andrewnguonly]

Summary

This PR resolves #27.

llava and bakllava are multimodal models available through Ollama. Images that are present on the current tab will be downloaded and bound to the model.

Implementation

1. Move getHtmlContent() to a separate scripts/content.ts file.
2. Update getHtmlContent() to retrieve src URLs from <img> elements inside the returned elements from the selectors and selectorsAll queries.
3. Update background script to download images and bind the base64 encoded image data to the model.