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

output.go

@@ -0,0 +1,63 @@
+package telegraf
+
+import "github.com/influxdata/influxdb/client/v2"
+
+// 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(metrics []Metric) 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(metrics []Metric) error
+// 	// Start the "service" that will provide an Output
+// 	Start() error
+// 	// Stop the "service" that will provide an Output
+// 	Stop()
+// }
+
+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()
+}