Introduce ~/.zsh/configs

Just before loading `~/.zshrc.local`, load:

1. `~/.zsh/configs/pre/**/*`
2. `~/.zsh/configs/**/*` # excluding pre and post
3. `~/.zsh/configs/post/**/*`

About the zsh glob:

- `.`: only produce normal files.
- `-`: follow symlinks to their final file; skip any broken links.
- `N`: do not complain about zero matches.

Big ups to Pat Brisbin for finding `N`.