Add new iptables plugin

The iptables plugin aims at monitoring bytes and packet counters
matching a given set of iptables rules.

Typically the user would set a dedicated monitoring chain into a given
iptables table, and add the rules to monitor to this chain. The plugin
will allow to focus on the counters for this particular table/chain.

closes #1471

etc/telegraf.conf

@@ -910,6 +910,18 @@
 #   ##
 #   servers = ["USERID:PASSW0RD@lan(192.168.1.1)"]
 
+# # Gather packets and bytes throughput from iptables
+# [[inputs.iptables]]
+#   ## iptables require root access on most systems.
+#   ## Setting 'use_sudo' to true will make use of sudo to run iptables.
+#   ## Users must configure sudo to allow telegraf user to run iptables.
+#   ## iptables can be restricted to only use list command  "iptables -nvL"
+#   use_sudo = false
+#   ## define the table to monitor:
+#   table = "filter"
+#   ## Defines the chains to monitor:
+#   chains = [ "INPUT" ]
+
 
 # # Read JMX metrics through Jolokia
 # [[inputs.jolokia]]