[completion] Fix known_hosts completion for custom port number (#1092)

Handles records like "[20.20.7.168]:9722 ssh-rsa ..."

This is a standard format for servers running on custom port according to http://man.openbsd.org/sshd.8#SSH_KNOWN_HOSTS_FILE_FORMAT

    A hostname or address may optionally be enclosed within ‘[’ and ‘]’
    brackets then followed by ‘:’ and a non-standard port number.