Add magnet link support

[xiongchiamiov]

As noted in this bug report, we don't currently support magnet links.

FWIW, the example CommonMark implementation handles this fine.

Up to you whether this is anything we're interested in supporting.

[jsayol]

I think it's worth adding support for magnet links, it adds functionality and should be a quick fix anyway.

Edit: if I'm not mistaken, it would be enough to add 'magnet:' here.
Edit 2: And/or here. First time looking at reddit's source code so I'm a bit lost :)

[jsayol]

By the way, here it says "mailto://" when it should be just "mailto:". I did a quick test and mailto links don't work right now.