Adding sli.mg support

[spiral6]

So, we have Imgur. It's great. But sometimes it goes down. And sometimes we hit a rate limit.

We can also look into creating an additional mirror, a backup if you will, to another site that acts very similarly. In this case, it's http://sli.mg.

If this is ok, I'll work on developing this export plugin as a backup to Imgur. Eventually, not only will this backup be good for the long run, but if we ever handle an Imgur exception, we can attempt to do a backup on this site and link it as an alternative mirror. This is an enhancement.

[Shugabuga]

I'm for sli.mg support. Should it act as an imgur fallback or as an alternate link (both Imgur and sli.mg will be shown in comment)?

[spiral6]

I think fallback would be good in the long run, it saves the bot work and reduces redundancy.

[kupiakos]

I'd probably say an Imgur fallback rather than creating both. I really would prefer to have the comments as small as possible.

[spiral6]

sli.mg doesn't work anymore. I'll keep thinking on another source.

[Shugabuga]

I did some brief research and these may work (IDK how good they are, just found them on /r/ImgurAlternatives and know they somewhat have APIs):

VGY

Biyori

BeeIMG