Added a formatter option

[dhendo]

Added a formatter option to allow the shape of the MongoDB log entry to be changed.

options.formatter = function(level, msg, meta) {
    return {
        datetime: new Date(),
        message: msg,
        tag: meta.tag
    };
}

This allows you to re-arrange and pull items out of the meta object to better fit your schema (handy if you have log entries coming in from elsewhere)

[indexzero]

This will be resolved in a future version of winston and should not be addressed in a single transport.