Add custom IP bind option (#425)

This PR adds an option in the server config to bind to a custom IP.

- [x] Review config comment and make sure that it is not confused with
port forwarding
- [x] Validate IP addresses

---

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include/Settings.h

@@ -79,6 +79,7 @@ struct Settings {
         General_Map,
         General_AuthKey,
         General_Private,
+        General_IP,
         General_Port,
         General_MaxCars,
         General_LogChat,