MongoDB driver: add optional authentication options

[novalagung]

In my experience, at least on two projects, the mongo optional authentication options need to be configurable.

Currently the authenticationDatabase are being hardcoded, and also it's not possible to pass the authenticationMechanism option.

This PR will solve this problem. I've updated the readme as well.

[novalagung]

#183

[novalagung]

Usage example:

connInfo := &db.ConnectionInfo{
    Host:     "localhost:27123",
    Database: "dbname",
    UserName: "",
    Password: "",
    Settings: tk.M{}.
        Set("authenticationDatabase", "adminuser").
        Set("authenticationMechanism", "SCRAM-SHA-256"),
}

conn, err := db.NewConnection("mongo", connInfo)
if err != nil {
    log.Fatal(err.Error())
    os.Exit(0)
}