Added basic banner option for shell

So, I'm using mongo-hacker in a few projects now, and I've added my own custom stuff to it. But I'm deploying it using puppet, and it would be useful to add a banner message to show that mongo-hacker is being loaded, and what version was being loaded.

Probably over-engineered by putting it into the config, but wasn't sure the best practise?

@TylerBrock what do you think? :+1: