Add a command that checks for missing translations in foreign language files

[elkuku]

This will add a command that checks foreign language translation files for missing strings.

Executing the command without arguments checks all language files:

Providing a locale parameter only files for this locale will be checked and untranslated keys are displayed along with their fallback message.

If everything is translated a <success> message will be displayed.

This command is using the SymfonyStyle class to style the console output using the "Symfony style guide". I believe that also the other console commands should use this class to ensure a uniform display.

[javiereguiluz]

@elkuku thanks for this contribution. However, I must close it as "won't merge". Why? Because Symfony already provides a command for this: debug:translation.

It's true that your command is more specialized and quickly provides a summary for all locales, but adding it could confuse Symfony developers into thinking that they must develop this command themselves in their projects. That's why I'm closing it. I'm sorry but I hope you understand it. Thanks!

[jkufner]

@elkuku I think this pull request should be posted to Symfony itself, rather than this demo app.