Support fallbacks

[adriandmitroca]

Hey @potsky,

What do you think about idea that accept null values as a translations? This approach would allow you to use translations fallbacks - because you probably want to display english translation to the user instead of something like TODO: hello_user

This could be possible using a custom parameter like:
php artisan localization:missing -l null or php artisan localization:missing --support-fallbacks

[potsky]

Thanx @adriandmitroca for the PR, it will be included in the next release.