Add support for setting custom document id's

[nodesocket]

Add the ability to provide a custom document id (aka key) to set using a new command specific option --key. This feature also bumps the major version to 2.0.0.

Any security implications? Make sure not able to override files outside of $JSONLITE_DATA_DIR.

Requires updating README.md, CHANGELOG.md, and adding new specific tests for this case as well.