Add IPCOMMAND directive

Added "IPCOMMAND" to provide a way to specify the IP without using curl.

nsupdate.sh

@@ -94,7 +94,11 @@ if ls $(dirname $0)/nsupdate.d/*.config &> /dev/null; then
       fi
 
       # WAN_IP=`curl -s -$CONNECTION_TYPE ${IP_CHECK_SITE}| grep -Eo '\<[[:digit:]]{1,3}(\.[[:digit:]]{1,3}){3}\>'`
-      WAN_IP=$(curl -s -$CONNECTION_TYPE ${IP_CHECK_SITE})
+      if [[ -n "$IPCOMMAND" ]]; then
+        WAN_IP=$($IPCOMMAND)
+      else
+        WAN_IP=$(curl -s -$CONNECTION_TYPE ${IP_CHECK_SITE})
+      fi
 
       # This is relevant for getting the specific domain record id.
       API_XML_INFO="<?xml version=\"1.0\"?>