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

internal/models/filter.go

@@ -1,4 +1,4 @@
-package models
+package internal_models
 
 import (
 	"strings"