Expose flag overrides to the router

As discussed in #14.

- Flags are stored in a cookie called `next-flags`
- Flags are extracted from the cookie and expose to the backend in the `x-flags` header.

Elsewhere,

- We can write something to set/unset the next-flags cookie.
- We can write some middleware to process these flags.
- We need to make sure applications vary on x-flags.