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

plugins/inputs/iptables/iptables_nocompile.go

@@ -0,0 +1,3 @@
+// +build !linux
+
+package iptables