Make NAT64 prefix a configurable option with default of well-known prefix

[nivex]

Initial plumbing to make the NAT64 prefix a configurable option. This is untested. I'm not a developer, just an interested user. I definitely do not know how to make the UI changes to present this to the user. Work toward fixing #13 .

[pmarks-net]

For this to become a proper feature, I would want to do IPv6 prefix parsing/matching, and format the address as prefix::a.b.c.d. But that's a lot more work, which is why I haven't done anything.

[treysis]

Maybe doing all at once seems too much...so implementing it stepwise sounds like a good solution to me.