Support inline images, source code, etc.

This may be a big one, and may warrant multiple separate issues. Here are some ideas:
- When a URL is pasted with a standard image format extension, it's downloaded and displayed inline
- When a URL from a popular image/video hosting site is pasted in, it's shown inline via that site's API
- When a link to a pasteboard is pasted in, the code is displayed inline -- via iframe?
