add support for custom regions, by detecting what is used in index.html (#3518)

read index.html to discover the regions used, make them the list checked
by app.js and check:config test

fixes #3504   supercedes #3506 

no config.js param required

tests/configs/customregions.js

@@ -0,0 +1,23 @@
+let config = {
+	modules:
+		// Using exotic content. This is why don't accept go to JSON configuration file
+		(() => {
+			let positions = ["row3_left", "top3_left1"];
+			let modules = Array();
+			for (let idx in positions) {
+				modules.push({
+					module: "helloworld",
+					position: positions[idx],
+					config: {
+						text: `Text in ${positions[idx]}`
+					}
+				});
+			}
+			return modules;
+		})()
+};
+
+/*************** DO NOT EDIT THE LINE BELOW ***************/
+if (typeof module !== "undefined") {
+	module.exports = config;
+}