Extract helpers.js (#130)

* Extract helpers.js
* Rebuild background.dist.js and popup.dist.js when helpers change
* Cleanup require definitions

src/helpers.js

@@ -0,0 +1,39 @@
+//------------------------------------- Initialisation --------------------------------------//
+"use strict";
+
+const TldJS = require("tldjs");
+
+module.exports = {
+    pathToDomain
+};
+
+//----------------------------------- Function definitions ----------------------------------//
+
+/**
+ * Get the deepest available domain component of a path
+ *
+ * @since 3.0.0
+ *
+ * @param string path        Path to parse
+ * @param string currentHost Current hostname for the active tab
+ * @return string|null Extracted domain
+ */
+function pathToDomain(path, currentHost) {
+    var parts = path.split(/\//).reverse();
+    for (var key in parts) {
+        if (parts[key].indexOf("@") >= 0) {
+            continue;
+        }
+        var t = TldJS.parse(parts[key]);
+        if (
+            t.isValid &&
+            ((t.tldExists && t.domain !== null) ||
+                t.hostname === currentHost ||
+                currentHost.endsWith(`.${t.hostname}`))
+        ) {
+            return t.hostname;
+        }
+    }
+
+    return null;
+}