Add settings for mirroring copied images to Stardown's server

[wheelercj]

Related: #79 and #81

When copying an image to be pasted into a remote editor like GitHub, 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 remote editors like GitHub may want Stardown to automatically mirror images and use the mirrored image URLs in image markdown so that image markdown continues working longer.

Most people won't want to set up a mirroring service themselves, but we could provide one for them. However, this could require a lot of time and effort spent moderating the images. Requiring users to create accounts could help. We should discuss this and maybe do some research.

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