Add HTTPData struct

http.go

@@ -0,0 +1,15 @@
+package step
+
+import "net/http"
+
+// HTTPData represents information about an ongoing HTTP request that is made
+// available to templates as they are being executed.
+type HTTPData struct {
+	// Res is like an http.ResponseWriter.
+	Res struct {
+		Header http.Header
+		WriteHeader func (statusCode int)
+	}
+	// Req is the HTTP request.
+	Req *http.Request
+}