Convert some URLs dragged into notepad to markdown of image, video, audio, etc.

[wheelercj]

The user can drag some content from websites into Stardown's notepad to make it appear in its markdown form there, but some things with a URL only appear in the notepad as the URL.

If the content dragged into the notepad is a URL, Stardown should try to determine whether it's the URL for an image, video, audio, etc. and automatically convert it to markdown of that.