Tidy up gitignore file

- `custom/example.zshcache` doesn't exist anymore: it's safe to
  ignore.

- `*.swp` files are ignored for no apparent reason. There's nothing
  in the codebase that generates them. This was introduced by @lepht
  in commit f78218f6 on 24 May 2011.

- Files `locals.zsh` and `projects.zsh` are no longer supported for
  putting custom configuration there.

  This is since commit 8debd097 where files were put in a lib/
  folder, but failed to modify the .gitignore file as well.

  The init script was modified as well, so those files aren't even
  being sourced: they are safe to be deleted or ported into a custom
  script.

.gitignore

-locals.zsh
-log/.zsh_history
-projects.zsh
-custom
+# custom files
+custom/
 !custom/plugins/example
 !custom/example.zsh
-*.swp
-!custom/example.zshcache
+
+# temp files directories
 cache/
-!cache/.gitkeep
 log/
-!log/.gitkeep