Implement a __proto__ check option

Eran Hammer posted an article on __proto__ poisoning and his solution in joi/hapi: https://hueniverse.com/a-tale-of-prototype-poisoning-2610fa170061

@rgrove posted a simple implementation of a fix for this: https://gist.github.com/rgrove/3ea9421b3912235e978f55e291f19d5d

However the fix requires a custom reviver that might slow down the default/valid parsing case, Eran prevented this by using an initial check for `__proto__`. It might be good to add this as a default to be checked for in body-parser in general that can be switched off... if someone wants to do so.