Add release assets as an output

[m4heshd]

This is an extension to #9.

It would be really useful if outputs contained something like assets which should be an array containing each response object from every asset upload. I think you can safely remove the uploader property which is unnecessary IMO.

I'm not sure about the data type though. I think it should be a JSON string which then the user can use with fromJSON(). Not sure if it's possible to set it as a proper sequence when setting the output. It's a valuable piece of data to have either way.

I'd do a PR myself but can't put the time aside at the moment. 🥵

[ncfavier]

I implemented this in #185