support : in autolinks

[matloob]

Edit: it looks like github does treat https://localhost as an autolink even though the GFM spec says a valid domain must have at least one dot. We should support https://localhost:8080

Program running goldmark and markdown on input: https://go.dev/play/p/FCJ0gp84YZQ

https://localhost:8080

output is ``` markdown:

https://localhost:8080

goldmark:

https://localhost:8080

It looks like goldmark is doing the wrong thing here but we should handle the :8080

[matloob]

This is a duplicate of #5