handler.js can return a Promise

[padiazg]

We should be able to handle when the handler.js defined by the user returns a promise in turn of a final value.

Expected Behaviour

The handler.js could return either a final value or a promise, and we should resolve it if so to get the value to return.

Current Behaviour

Now the handler.js exports a function that receives a callback to return a resulting value, with this improvement the user could return also a promise.