Support for IPv6 and CIDR in black-/whitelist

Hi,

with most of my sites running on native Dual Stack servers and increasing numbers of IPv6 clients black- and whitelists should support IPv6 addresses.

I had a look into the code and apparently the use of ip2long() does not support that, so simply entering a range like 2001:db8::1-2001:db8:ffff:ffff:ffff:ffff:ffff:ffff does not lead to any valid result.

Please consider adding support for that.

A bonus would be CIDR notation support, s.t. the above example can be reduced to 2001:db8::/32.

I’ve added a Gist for migration of the existing code to inet_pton (which supports IPv6 if PHP supports it):
https://gist.github.com/stklcode/59447dd8419ad79d2bcd5d38d733ea65

For CIDR match an example implementation can be found in one of my plugins (required some boilerplate to be compatible with current range notation):
https://github.com/stklcode/statify-blacklist/blob/f14ceac40081617cef1d588ea725f1f8d5fb3117/inc/class-statifyblacklist.php#L294-L365

Cheers,
Stefan