Allow discovery of internal NAT64 prefixes

[hb9hnt]

FIrst of all: thank you for this awesome extension!

Currently I'm having one issue with it: We use a separate internal NAT64 prefix within our IPv6 block for internal connections. Because the list of URLs to discover the prefixes not configurable, these connections show up as native IPv6 connections.

It would be nice if I could either configure a separate NAT64 prefix or a custom URL to discover prefixs (i.e. ipv4.myinternaldomain.com).

It's an absolute nice to have but it would help me see right away what we have not migrated internally.

[pmarks-net]

Here's a workaround:

• In Chrome, right-click the IPvFoo popup > Inspect
• In Firefox, go to about:debugging and find the Inspect button
• Go to the Console tab and type addNAT64('myprefix::')

At some point, I might decide to build this into the UI, but that requires a bunch of features (selection/add/remove/validation)

[hb9hnt]

Awesome! Thanks for the workaround!