addHandler(cb) {
    if (!_process) {
      throw new Error('process is not captured');
    }

    let index = handlers.indexOf(cb);
    if (index > -1) {
      return;
    }

    if (handlers.length === 0) {
      setupSignalsTrap();
    }

    handlers.push(cb);
    captureExit.onExit(cb);
  }
Add process interruption handler

When the first handler is added then automatically
sets up process interruption signals listeners

@private
@method addHandler
@param {function} cb   Callback to be called when process interruption fired