Nether Portal-style linking with SignPortals

[kherock]

It would be really nice if Portal-style SignPortals could use the same linking mechanisms as overworld-to-nether portals did for each other. This would include spawning another Nether Portal with a sign when there are no portals in range.

I believe this would actually be a straightforward port from a feature that is already in MV-NetherPortals. The reason I am not using that plugin is because I still like to have two-way access to the nether.