Make timeagoFormatter optional

src/core/client/framework/lib/bootstrap/TalkContext.tsx

@@ -13,7 +13,7 @@ export interface TalkContext {
   localeMessages: MessageContext[];
 
   // formatter for timeago.
-  timeagoFormatter: Formatter | null;
+  timeagoFormatter?: Formatter;
 }
 
 const { Provider, Consumer } = React.createContext<TalkContext>({} as any);