Enable specifying which day to run

I like this tradeoff of running a single day in isolation so the output isn't muddied by other days but also being able to go back and run previous days' solutions (for comparison with friends also participating in advent). The only thing I wish I could do here is run "all" to fire them back to back, but this particular implementation doesn't allow that easily. Maybe later...

src/main.cs

@@ -1,8 +1,39 @@
-//aoc2021.Day02.Go();
-//aoc2021.Day03.Go();
-//aoc2021.Day05.Go();
-//aoc2021.Day06.Go();
-//aoc2021.Day07.Go();
-//aoc2021.Day08.Go();
-//aoc2021.Day09.Go();
-aoc2021.Day10.Go();
+var arg = args.FirstOrDefault();
+switch (arg)
+{
+    case "1":
+        aoc2021.Day01.Go();
+        break;
+
+    case "2":
+        aoc2021.Day02.Go();
+        break;
+
+    case "3":
+        aoc2021.Day03.Go();
+        break;
+
+    case "5":
+        aoc2021.Day05.Go();
+        break;
+
+    case "6":
+        aoc2021.Day06.Go();
+        break;
+
+    case "7":
+        aoc2021.Day07.Go();
+        break;
+
+    case "8":
+        aoc2021.Day08.Go();
+        break;
+
+    case "9":
+        aoc2021.Day09.Go();
+        break;
+
+    default:
+        aoc2021.Day10.Go();
+        break;
+}