Emit warning when loading a gem that isn't bundled

[nogweii]

I ran into the situation described in the README:

Before you rush off to try this, a word of warning: you will waste debugging time because of this. Why? Because running a require 'ampex' inside Pry works, but running a require 'ampex' outside Pry doesn't. "XOMGWTF? Ohhhh! GAH!!" I hear your future self cry as you forget this warning, and then painfully recall it.

So, is there a way to tell when a requested gem is not already part of the bundle and emit a warning when it's been requested? It would have indeed saved a lot of time...

[ConradIrwin]

Not trivially.... If you can figure out a way, I'd merge it.