re-use live mode config - and improve docs on how to start liveMode (#1003)

* Use config.liveMode to turn on liveMode in the timer

* Discussion of `startLive(1500)` was incorrect as startLive is based on timestamps, not playback offsets.  Also reorder to introduce `addEvent` earlier

Co-authored-by: Eoghan Murray <eoghan@getthere.ie>

packages/rrweb/src/replay/index.ts

@@ -262,7 +262,10 @@ export class Replayer {
       this.mirror.reset();
     });
 
-    const timer = new Timer([], config?.speed || defaultConfig.speed);
+    const timer = new Timer([], {
+      speed: this.config.speed,
+      liveMode: this.config.liveMode,
+    });
     this.service = createPlayerService(
       {
         events: events

packages/rrweb/src/replay/timer.ts

@@ -13,9 +13,16 @@ export class Timer {
   private raf: number | null = null;
   private liveMode: boolean;
 
-  constructor(actions: actionWithDelay[] = [], speed: number) {
+  constructor(
+    actions: actionWithDelay[] = [],
+    config: {
+      speed: number;
+      liveMode: boolean;
+    },
+  ) {
     this.actions = actions;
-    this.speed = speed;
+    this.speed = config.speed;
+    this.liveMode = config.liveMode;
   }
   /**
    * Add an action after the timer starts.