Add TypeScript support

[pke]

I wonder how one could make this a little bit more type-safe by having the middleware actually know the types of the arguments and the supposed return value (Promose<Response>).

I have started with this:

// This should be exported by "fetch-wrap" as a type
type FetchFunction = typeof fetch

export function logHeaders() {
  return function(url: string, options:RequestInit = {}, fetch: FetchFunction):Promise<Response> {
  }
}