impl #81 custom event

This is the record side impl of custom event, according to the
issue, we may also add first-class support for the custom event
tag like display color labels in the replayer-ui.

src/index.ts

@@ -1,6 +1,11 @@
-import record from './record';
+import record, { addCustomEvent } from './record';
 import { Replayer } from './replay';
 import { mirror } from './utils';
 
-export { EventType,  IncrementalSource, MouseInteractions, ReplayerEvents } from './types'
-export { record, Replayer, mirror };
+export {
+  EventType,
+  IncrementalSource,
+  MouseInteractions,
+  ReplayerEvents,
+} from './types';
+export { record, addCustomEvent, Replayer, mirror };