[WIP] feat: support es6 module default export

[tungv]

this allows projects that compiled through babel can conveniently export default a Promise

// instead of
module.exports = app.prepare().then(() => service);

// babel users can do this
export default app.prepare().then(() => service);

[leo]

Wonderful addition! Can you add tests?

[tungv]

@leo I'm not sure which test file is in charge of this kind of functionality currently. Probably I need to add tests for handler.js

[tungv]

@leo I'm not sure how to test this, can you point me to the test for async setup function so I can have a clue?

[timneutkens]

This is perfect 👌

[timneutkens]

@tungv I'll create an issue for the tests. I'll add them 👍

[tungv]

thank you @timneutkens. As a heavy babel user, I'm longing for this feature