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

cache/.easter-egg

-This file is only here so that Git will keep a cache directory as .gitignore is ignoring all the files within it.
-
-Feel free to add love notes for people here.
-

log/.easter-egg

-This file is only here so that Git will keep a log directory as .gitignore is ignoring all the log files within it.
-
-feel free to add love notes for people here.
-