Custom logging missing translation with Unit tests

[DServy]

Updates for Missing Translations, adds a passthrough that a user can run a function hook to do something with the missing function. Already proven super helpful for some logging stuff we need to do. In theory can be used with a Zanata instance to automatically add in missing translations. Possibilities are huge.

example usage:

missingTranslation: function(locale, value){
        value = value.replace('\n', ' ');
        console.log("app='%s' type='MissingTranslation' language='%s' string='%s'", process.env.HEROKU_APP_NAME, locale, value);
    }

Updates to make pull #233 unit tests work for @farfromrefug. Fixed merge conflicts, making the unit test pass

[coveralls]

Coverage increased (+0.02%) to 97.925% when pulling 48e7afb on DServy:custom-logging-missing-translation into 11d89da on mashpie:master.

[coveralls]

Coverage increased (+0.02%) to 97.925% when pulling 48e7afb on DServy:custom-logging-missing-translation into 11d89da on mashpie:master.

[coveralls]

Coverage increased (+0.02%) to 97.925% when pulling 48e7afb on DServy:custom-logging-missing-translation into 11d89da on mashpie:master.

[njroussel]

This is really necessary ! Can we get a new version published once this is merged ?

[mashpie]

now there is generic config option to setup your own callback function:

    // used to alter the behaviour of missing keys
    missingKeyFn: function (locale, value) {
        return value;
    },

you may add logging, return custom values or anything.