carbonapi: allow to configure CORS options

[avereha]

I'm particularly interested in :

https://github.com/gorilla/handlers/blob/master/cors.go#L193

We seem to use the CORS middleware:

https://github.com/bookingcom/carbonapi/blob/master/app/carbonapi/app.go#L107

But we do not allow configuring it.

[haseth]

Is anyone working ? Or can I pick it up? @avereha @azhiltsov

[avereha]

No one is working on it, feel free to pick it up.