Create public models for telegraf metrics, accumlator, plugins

This will basically make the root directory a place for storing the
major telegraf interfaces, which will make telegraf's godoc looks quite
a bit nicer. And make it easier for contributors to lookup the few data
types that they actually care about.

closes #564

plugins/outputs/registry.go

@@ -1,40 +1,10 @@
 package outputs
 
 import (
-	"github.com/influxdata/influxdb/client/v2"
+	"github.com/influxdata/telegraf"
 )
 
-type Output interface {
-	// Connect to the Output
-	Connect() error
-	// Close any connections to the Output
-	Close() error
-	// Description returns a one-sentence description on the Output
-	Description() string
-	// SampleConfig returns the default configuration of the Output
-	SampleConfig() string
-	// Write takes in group of points to be written to the Output
-	Write(points []*client.Point) error
-}
-
-type ServiceOutput interface {
-	// Connect to the Output
-	Connect() error
-	// Close any connections to the Output
-	Close() error
-	// Description returns a one-sentence description on the Output
-	Description() string
-	// SampleConfig returns the default configuration of the Output
-	SampleConfig() string
-	// Write takes in group of points to be written to the Output
-	Write(points []*client.Point) error
-	// Start the "service" that will provide an Output
-	Start() error
-	// Stop the "service" that will provide an Output
-	Stop()
-}
-
-type Creator func() Output
+type Creator func() telegraf.Output
 
 var Outputs = map[string]Creator{}