Add IPv6 support for port forwarding and self-injection

Description / Changes / Goal

Description

Currently only IPv4 address can be specified for port forwarding and self-injection.
So added IPv6 support for them like below.

$ pwncat -l 4444 --self-inject /bin/bash:[xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx]:4445
$ pwncat -R [xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx]:4444 10.0.0.1 3306
$ pwncat -L [xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx]:5000 10.0.0.1 3306

Changes

Modified arguments parsing for those options to accept IPv6 address.

Goal

Be able to specify IPv6 address for --self-inject/-R/-L by surrounding the address with [].

PR Checklist

• GitHub issue linked to this PR?: IPv6 self injection / port forwarding #88
• CHANGELOG.md updated?
• Run make lint?
• Run make code?