Update $i18n.keyExists method
Based on #57 the method $i18n.keyExists() was slightly modified.
Prior to v1.9.0, the method would check for existence of the key in the current locale and the fallback locale.
Now, the method will check the current locale, the regional parent language locale (i.e. en for en-us) and the fallback locale for the key. This is in line with how the method $t() will translate a given key.
Additionally a second parameter can be specified to limit the scope that should be checked for the key $i18n.keyExists('key', 'strict'). Available options are strict (only current locale), locale (current locale and regional parent language) and fallback (current locale, regional parent locale and fallback locale). Details can be found in the readme.