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 8debd09 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.