Extract all config constants in one interface

[Riduidel]

Currently, config values are scattered all across the application, which makes their finding difficult.
What makes it worse is they are only present as String value.

Example with the code added for #30

    if (config.getString("default.status") != null) {
        // default status has been set
        if (content.get("status") == null) {
            // file hasn't got status so use default
            content.put("status", config.getString("default.status"));
        }
    }

i suggest moving all these constants into a Config interface, defined as public static inner interface of ConfigUtils. This way they're both easier to locate, and to document.

[jonbullock]

Thanks for raising an issue for this, the values should be centralised somewhere.