Update i18n.js

Added option missingBehavior, which avoids to have to create strings when developing or simple strings that do not need a declaration in the default language.

- move options comments at declaring array, as this is the right place
- recursive declaration of reset and initializeOptions
- added 'missingBehavior' option with possible values 'message' (or anything else) keeping the actual behavior, 'guess' tries to create a readable string