Add demo for server middleware extensibility

As per RFC 0005: SAP/ui5-tooling#151

middleware/ponyTranslator.js

@@ -0,0 +1,29 @@
+const rProperties = /\.properties$/;
+const parseurl = require("parseurl");
+
+module.exports = function({resources, options}) {
+	let replacement = "pony"
+	if (options.configuration.mood) {
+		replacement = `${options.configuration.mood} ${replacement}`;
+	}
+	return function (req, res, next) {
+		const pathname = parseurl(req).pathname;
+		if (!rProperties.test(pathname)) {
+			next();
+			return;
+		}
+		resources.all.byPath(pathname).then(function(resource) {
+			if (!resource) { // Not found
+				next();
+				return;
+			}
+
+			resource.getBuffer().then((content) => {
+				const ponianizedContent = content.toString().replace(/=[^=]*$/gm, `=${replacement}`);
+				res.end(ponianizedContent);
+			});
+		}).catch((err) => {
+			next(err);
+		});
+	}
+};

ui5.yaml

@@ -2,3 +2,18 @@ specVersion: '1.0'
 metadata:
   name: openui5-sample-app
 type: application
+server:
+  customMiddleware:
+    - name: ponyTranslator
+      beforeMiddleware: serveResources
+      mountPath: /
+      configuration:
+        mood: "🐤 happy"
+---
+specVersion: '1.0'
+kind: extension
+type: server-middleware
+metadata:
+  name: ponyTranslator
+middleware:
+  path: middleware/ponyTranslator.js