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

registry/outputs/registry.go

@@ -0,0 +1,13 @@
+package outputs
+
+import (
+	"github.com/influxdata/telegraf"
+)
+
+type Creator func() telegraf.Output
+
+var Outputs = map[string]Creator{}
+
+func Add(name string, creator Creator) {
+	Outputs[name] = creator
+}