chore(growth): allow one to customize Req options

With this change it's possible to change the way the application make
requests to WHO website.

lib/growth/indicators/download.ex

@@ -240,7 +240,16 @@ defmodule Growth.Indicators.Download do
   end
 
   def fetch!(url) do
-    case Req.get(url) do
+    req =
+      [url: url]
+      |> Keyword.merge(
+        :wabanex
+        |> Application.get_env(__MODULE__, [])
+        |> Keyword.get(:who_req_options, [])
+      )
+      |> Req.new()
+
+    case Req.get(req) do
       {:ok, %{status: 200, body: body}} ->
         body