Gently warn about global variables

Quite often, beginners or even seasoned developers get tripped up by something that is already defined in the global scope. It's a beautiful thing to try and debug - happened to me recently with `window.name`, for example. 

I'm not 100% on the wording, and totally aware that discussing globals is skipping ahead a little, but when people find something isn't working as they expect, then warning them about the environment not being empty seems like a good idea.