Fix #371, make unpause resume reading an about:reader page

Generally this makes Reader Mode narration a kind of service

extension/services/readerMode/readerMode.js

@@ -0,0 +1,36 @@
+/* globals intents, serviceList, content */
+
+this.services.readerMode = (function() {
+  class ReaderMode extends serviceList.Service {
+    async pause() {
+      await this.initTab("/services/spotify/player.js");
+      await this.callTab("pause");
+    }
+
+    async unpause() {
+      const activeTab = (await browser.tabs.query({ active: true }))[0];
+      if (!activeTab.url.startsWith("about:reader")) {
+        const e = new Error("Cannot unpause a non-reader tab");
+        e.displayMessage = "Cannot unpause";
+        throw e;
+      }
+      await content.lazyInject(activeTab.id, [
+        "/intents/read/startNarration.js",
+      ]);
+      await this.callOneTab(activeTab.id, "narrate");
+    }
+
+    async pauseAny() {
+      return intents.read.pauseAny();
+    }
+  }
+
+  Object.assign(ReaderMode, {
+    id: "readerMode",
+    title: "Reader Mode",
+    baseUrl: "about:reader",
+    skipAutodetect: true,
+  });
+
+  intents.music.register(ReaderMode);
+})();