add get_input task

call with `mix get_input <year> <day>` and it
will write to `input/<year>_<day>.txt`

lib/get_input.ex

@@ -0,0 +1,23 @@
+defmodule Mix.Tasks.GetInput do
+  defp get_session() do
+    {_, content} = File.read("./.session")
+    content |> String.trim()
+  end
+
+  defp get_request_headers() do
+    [Cookie: "session=#{get_session()}"]
+  end
+
+  def run(args) do
+    [year, day] = args
+    write_file = &File.write("./input/#{year}_#{day}.txt", &1)
+    HTTPoison.start
+    HTTPoison.get!(
+      "https://adventofcode.com/#{year}/day/#{day}/input",
+      get_request_headers()
+    )
+    |> Map.get(:body) 
+    |> then(fn i -> IO.puts(i); i end)
+    |> write_file.()
+  end
+end