Implement a minimal config object

[ax1]

new Db(file,{options})

where one of the option is the name of the id field, by default id, but it could be _id or name or whatever.

Note that this is really optional, because currently, we do not need any config values and if we start with that we would end up with several config params (and I prefer standard conventions over config because it is straightforward to use it without digging in documentation every time.

The shorter the doc, the better. Look at a1-server, even if the docum is pretty small for a full http server (and that has been difficult to achieve), it is big to my eyes.