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/inputs/registry.go

@@ -0,0 +1,11 @@
+package inputs
+
+import "github.com/influxdata/telegraf"
+
+type Creator func() telegraf.Input
+
+var Inputs = map[string]Creator{}
+
+func Add(name string, creator Creator) {
+	Inputs[name] = creator
+}