Support Processor & Aggregator Plugins

closes #1726

plugins/aggregators/registry.go

@@ -0,0 +1,11 @@
+package aggregators
+
+import "github.com/influxdata/telegraf"
+
+type Creator func() telegraf.Aggregator
+
+var Aggregators = map[string]Creator{}
+
+func Add(name string, creator Creator) {
+	Aggregators[name] = creator
+}