Add/recommend rate-limiting middleware

[EvanHahn]

I'd love this:

app.use(helmet.rateLimit({
  rate: 500,     // 500 requests allowed every...
  window: 50000, // ...50 seconds
  whitelist: ["127.0.0.1"]
});

Could be included by default.

[m13]

I guess it could be a bit tricky, as sometimes there is a cluster of machines.

[EvanHahn]

Agreed; perhaps we should recommend someone else's middleware rather than make our own.

[EvanHahn]

Because there's so much rate-limiting middleware, I'm going to close this and just not do it.