Adding required parameter bang notation support (will raise Configatron:...

We use configatron extensively in our Rails apps and very much like its simplicity and flexibility, but have been bitten on a few occasions by missing/misconfigured mandatory environment configuration parameters (e.g. a URL to access an external service). Rather than failing fast you get more difficult to trace runtime failures when the `Configatron::Store` instance returned is used as a URL. Being environment specific, this is also the kind of thing your tests don't tend to pick up. 

Yes you can manually check if a given parameter is configured at runtime before using it, but this just makes it more convenient by simply appending a bang when the parameter must be there.