Insecure default permissions for history file

The history file [gets created with default permissions](https://github.com/chzyer/readline/blob/fcb4d7d9a9f653462a7adf557fb1f931f00391f2/history.go#L69) (before applying umask) of `0666`, which, assuming a default umask of 022, leaves the history file world readable. This could result in sensitive information being exposed. A better default would be `0660`.