Use .gitkeep files to keep cache/ and log/ directories

Instead of .easter-egg files we now have empty .gitkeep files,
like is done as an informal standard.

.gitignore

@@ -7,3 +7,6 @@ custom
 *.swp
 !custom/example.zshcache
 cache/
+!cache/.gitkeep
+log/
+!log/.gitkeep