Add S3 event source

[jcampbell05]

It would be great if either the S3 event source was added or if there was just a way to manually map an event to an event type and then can just pass in the route config like we can with the built in types

[brettstack]

You can create a custom mapper. See https://github.com/vendia/serverless-express/tree/mainline/examples/custom-mapper-dynamodb for an example

[brettstack]

Happy to take PRs :)

[jcampbell05]

Will this allow the built events to be consumed or does it override ?

[jongreenwood-dunelm]

I have created a PR for this #632