Fix printing ipv6 with brackets

[Felixoid]

It was fixed in #42, but is broken now. This PR prints ipv6 in the format, which can be opened in a browser:

>±> ./gotty zsh
2019/12/30 13:54:18 GoTTY is starting with command: zsh
2019/12/30 13:54:18 HTTP server is listening at: http://[::]:8080/
2019/12/30 13:54:18 Alternative URL: http://127.0.0.1:8080/
2019/12/30 13:54:18 Alternative URL: http://[::1]:8080/
2019/12/30 13:54:18 Alternative URL: http://[fe80::2ba1:dd5e:2dd9:9d75]:8080/
2019/12/30 13:54:18 Alternative URL: http://172.0.0.207:8080/
2019/12/30 13:54:18 Alternative URL: http://[fe80::f81d:ef8:fea5:f9f5]:8080/

[sorenisanerd]

Cool!
Merged at https://github.com/sorenisanerd/gotty