Passing SIGINT signals inside the application.

## Prerequisites

* [x] I am running the latest version. (`up upgrade`)
* [x] I searched to see if the issue already exists.
* [ ] I inspected the verbose debug output with the `-v, --verbose` flag.
* [ ] Are you an Up Pro subscriber?

## Description
I have an express application wrapped by `up`.
I have following code:
```js
process.on('SIGINT', () => {
// Do some actions
});
```
As far as I know, the up is triggered once request reaches lambda.
And any SIGINT signal received, reaches the `up` proxy, not the express application.

Is there any way to make the signal reach the application code as well?