move getIP function to its own module

extra_modules/getip.js

@@ -0,0 +1,12 @@
+/**
+ * gets ip of a request
+ * @param {request} req 
+ * @returns ip of the given request, after taking preferred headers into account
+ */
+ function getIP(req) {
+    let ip = req.socket.remoteAddress;
+    if(req.headers[config.preferred_ip_header] != undefined && ip == config.only_prefer_when_ip)ip = req.headers[config.preferred_ip_header]
+    return ip
+  }
+
+module.exports = getIP