break telegraf registry into separate package

this is for supporting external plugins.

external plugins will depend on a few telegraf interface types, as well
as a common telegraf registry.

this will allow external and internal plugins to both share this package
and make it easier to vendor/version the whole thing semantically, which
will make it easier to keep plugins supported across build and telegraf
versions.

see #1717

plugins/inputs/snmp_legacy/snmp_legacy.go

@@ -9,7 +9,7 @@ import (
 	"time"
 
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 
 	"github.com/soniah/gosnmp"
 )