Fail or warn if a message key is not used anywhere in the code

[amire80]

It might be useful to run something like git grep in all code files (probably *.php, *.js, maybe *.rb) and to issue a warning or a fatal failure if the message key cannot be found anywhere.

Such a thing can happen if a feature that uses the message was removed or changed and the developer forgot to remove the messages from en.json.

[Krinkle]

A few ideas:

• Build a single regex that matches all keys.
• Use the "async" package to provide high concurrency and asynchronously read each file to match (e.g. async.eachLimit).
• Track each match (e.g. seen[key] = true; or inverse like delete notSeen[key];).
• Report the remaining ones. (e.g. Object.keys(notSeen);)