Add settings for downloading copied images

[wheelercj]

Related: #80 and #81

When copying an image, Stardown only gets the image's URL, not the image itself. This works, but will eventually stop working when the image is no longer served by that URL. Those using local editors like Obsidian may want Stardown to download images, automatically put them in a folder of their choice, and include the file path to the downloaded image in the image markdown.

This will require another browser extension permission, but the permission can be optional and only requested if and when the user turns on image downloading in Stardown's settings.

This feature will require two new settings:

• whether to download images (default to not downloading)
• where to download images to

[wheelercj]

If the downloaded images must be in the downloads folder, they should be put into a folder within the downloads folder, such as downloads/stardown/image1.png.