Ability to add more than one url as image source

[HushedFox]

Is your feature request related to a problem? Please describe.
Usually artists post their art to more than one site and it's nice to be able to have links to multiple sites, especially because some of them might require you to log in to browse but others won't or urls might become dead in the future (like it was with NSFW tumblr content).

Describe the solution you'd like
Add more sources than just one to an image

Describe alternatives you've considered
Manually placing sources in description of the image. But there is a major problem with that - only staff and the uploader can do that, not anyone else like with editing sources.

Additional context
I'm just surprised GitHub issue about this was not already here months ago. I never added one because I though there was one.

[TheSmilingPony]

See derpibooru#101