[Request] Better IPv6 support

[Kagee]

Is your feature request related to a problem? Please describe.
The current IPV6 support is limited to a single IPv6 CIDR, and written with (unusual) large characters. In a network with proper IPv6 support (not just link-local), browser are free to select any of the avaliable IPv6 adresses of a host for connections. Thus allowing just link-local adresses are not enough.

Describe the solution you'd like
If possible, i would suggest re-using the MIT licensed code from https://github.com/sindresorhus/ip-regex.
CIDR support could be implemented by using the code in https://github.com/silverwind/cidr-regex, or by splitting the input string on /, making sure there are only two groups, and making sure the second group is [0 - 32] for IPv4, and [0-128] for IPv6.

If you require any help for testing or have questions, I may be able to help.

[cp2004]

I can give something like this a go in the future, it is not necessarily high priority - you can bypass validation by adding a valid configuration, then edit it - the plugin was deliberately designed this way.

If you have good ideas to adjust the current validation, a PR would be awesome - I don't currently have the time for this, but it is still on my mind. I might have created too many things to keep up with...