Support multiple local addresses on the same NAT

[Stebalien]

While not critical, it would be nice if this library supported multiple interfaces/addresses on the same NAT. This would mean:

1. Passing a local address (and using it as the source address) when establishing a mapping.
2. Changing GetInternalAddress() to GetInternalAddresses() (or adding a new function) that exposes all internal addresses on the NAT.

Use-cases:

• Multiple "virtual" interfaces (e.g., for containers/VMs).
• Multiple NICs on the same LAN.

I understand you probably don't have time to work on this, I just figured I'd leave this here in case anyone comes along and does.