Support Processor & Aggregator Plugins

closes #1726

aggregator.go

@@ -0,0 +1,16 @@
+package telegraf
+
+type Aggregator interface {
+	// SampleConfig returns the default configuration of the Input
+	SampleConfig() string
+
+	// Description returns a one-sentence description on the Input
+	Description() string
+
+	// Apply the metric to the aggregator
+	Apply(in Metric)
+
+	// Start starts the service filter with the given accumulator
+	Start(acc Accumulator) error
+	Stop()
+}