Added support for keep_secret parameter, preventing output of actual values to logs

Typical use case is to use this module for managing some sensitive values, like passwords, which we do not want in the logs. Using this new parameter `keep_secret` will prevent this.

Accepted values:
* `false` - default, normal behavior
* `true` - value will be replaced with "[sorry, this is secret]"
* `md5` - value will be replaced with it's md5 hash