added option for logErrors

Unless I haven't figured out how to catch errors properly, 
any caught error prevents the task from finishing, so it never re-fires on next file save.  

This is my gulpfile:
```javascript
gulp.task('es6', function () {
  return gulp.src(paths.server)
    .pipe(changed(paths.build))
    .pipe(traceur(traceurOpts).on('error', handleError))
    .pipe(gulp.dest(paths.build));
});
```

**Adding this option for `logErrors: true` works great.**