add vendor

vendor/github.com/mattn/go-isatty/isatty_appengine.go

@@ -0,0 +1,9 @@
+// +build appengine
+
+package isatty
+
+// IsTerminal returns true if the file descriptor is terminal which
+// is always false on on appengine classic which is a sandboxed PaaS.
+func IsTerminal(fd uintptr) bool {
+	return false
+}