add go-plugin deps to vendor

vendor/github.com/hashicorp/go-plugin/stream.go

@@ -0,0 +1,18 @@
+package plugin
+
+import (
+	"io"
+	"log"
+)
+
+func copyStream(name string, dst io.Writer, src io.Reader) {
+	if src == nil {
+		panic(name + ": src is nil")
+	}
+	if dst == nil {
+		panic(name + ": dst is nil")
+	}
+	if _, err := io.Copy(dst, src); err != nil && err != io.EOF {
+		log.Printf("[ERR] plugin: stream copy '%s' error: %s", name, err)
+	}
+}