Allow command to run this on the background

[mathieudevos]

Issue was mentioned earlier in #15

However would it be possible to add a --detached mode which would run the server in the child, and exit the process, thus freeing up the "terminal".

This would allow many CI/CD pipelines to run this at the start of the the tests such as:
await dynamoDbLocal.launch(8000, null, ['-detached']);
... run all the tests
await dynamoDbLocal.stop(8000);