Add code to fetch input from AoC.

src/advent/input.cr

@@ -0,0 +1,15 @@
+require "http/client"
+require "uri"
+
+def input(year, day)
+  filename = "year#{year}day#{day}.txt.cache"
+  return File.read(filename) if File.exists? filename
+  HTTP::Client.new(URI.parse("https://adventofcode.com")) do |client|
+    client.before_request do |request|
+      request.cookies["session"] = ENV["AOC_SESSION"]
+    end
+    input = client.get("/#{year}/day/#{day}/input").body.to_s
+    File.write(filename, input)
+    return input
+  end
+end