repl: make REPLServer emit an "exit" event

This can happen when "SIGINT" is processed by the readline interface,
or when the user types ".exit" into the REPL.

lib/repl.js

@@ -140,7 +140,9 @@ function REPLServer(prompt, stream, eval, useGlobal, ignoreUndefined) {
   rli.on('SIGINT', function() {
     if (sawSIGINT) {
       rli.pause();
-      process.exit();
+      self.emit('exit');
+      sawSIGINT = false;
+      return;
     }
 
     rli.line = '';
@@ -766,6 +768,7 @@ function defineDefaultCommands(repl) {
     help: 'Exit the repl',
     action: function() {
       this.rli.pause();
+      this.emit('exit');
     }
   });