Added compose script.

extension/compose.js

+// Modify the displayed message to clarify where safelinks are
+// inserted and to show the actual link targets.
+
+for (const link of document.links) {
+    if (isTangledLink(link.href)) {
+	link.href = untangleLink(link.href);
+    }
+}
+
+for (const node of getTextNodes(document)) {
+    node.textContent = untangleLink(node.textContent);
+}
+