Allow additional properties in manifest.json

[mohsen1]

The manifest.json is used for a lot of things and it's hard to add new properties for each new item vendors add to it.

Right now we need gcm_sender_id, related_applications and prefer_related_applications fields in our manifest.json.

It would be nice if there was an option for adding new properties to manifest.json. It can use Object.assign or a deep extend to add those properties.

[alexander-akait]

@mohsen1 Why don't use response and add properties that you need?

[ozasadnyy]

@evilebottnawi could you add some example how to do that? But in my opinion, it would be easier just to pass custom properties via settings.