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/mqtt/mqtt.go

@@ -13,6 +13,7 @@ import (
 	"github.com/influxdata/influxdb/client/v2"
 	"github.com/influxdata/telegraf/internal"
 	"github.com/influxdata/telegraf/plugins/outputs"
+	"github.com/influxdata/telegraf"
 )
 
 const MaxClientIdLen = 8
@@ -184,7 +185,7 @@ func getCertPool(pemPath string) (*x509.CertPool, error) {
 }
 
 func init() {
-	outputs.Add("mqtt", func() outputs.Output {
+	outputs.Add("mqtt", func() telegraf.Output {
 		return &MQTT{}
 	})
 }