Hide all files, add to clean script

.gitignore

@@ -13,3 +13,6 @@
 # Haskell IR files
 *.hi
 *.o
+
+# Graphing images created with Cairo
+*.png

flake.nix

@@ -65,7 +65,7 @@
             find . -type f -name '*.o' -delete
 
           # Delete any test graphs created
-            find . -type f -name 'test.png' -delete
+            find . -type f -name '*.png' -delete
         '';
       in
       pkgs.mkShell {