Adding saving current live database to timestamped JSON file

I always wanted to be able to save the current live db into a file, like nodemon's restart on 'rs'.
I added listening to user keyboard input. Whenever the user enters 's' or 'S' the json-server will dump the current live db into a timestamped file json-server.date-time.json.

I have not written tests for this, but will be happy to do so, if needed. 
It is setup for read-write.js routes, but would be very simple to add for read-only.js (should be same data though)