URLs with IPv6 addresses
Not sure how many variations we want to support, but at a minimum I think we should be able to correctly link with scheme like so:
Bare IPv6 addresses with or without the square brackets would be nice, but probably not worth it. Then we have to make sure we aren't linking an address/prefix... headaches galore. We already have a neat problem with #17
It's funny that GitLab doesn't get this correct either :)