Expose functions at API layer

Creating a simple HTTP RESTful API for exposing our `Server.semiprime`
function. It supports some help messages, primitive parsing and error handling,
and singular vs. batch processing of arguments.

For more sophisticated parsing and error-checking, I prefer to use Haskell's
Servant library.

assessments/semiprimes/server/lib/sup.ex

@@ -5,6 +5,7 @@ defmodule Sup do
   """
 
   use Supervisor
+  alias Plug.Adapters.Cowboy
 
   def start_link(opts \\ []) do
     Supervisor.start_link(__MODULE__, :ok, opts)
@@ -13,7 +14,8 @@ defmodule Sup do
   @impl true
   def init(:ok) do
     children = [
-      Cache
+      Cache,
+      Cowboy.child_spec(scheme: :http, plug: Router, options: [port: 8000])
     ]
 
     Supervisor.init(children, strategy: :one_for_one)