Document vim-mkdir in README
Use `gh`, not `hub`

[gh] is a [hub] reimplementation that's much faster and is now the official
Github CLI. It appears that "hub" is [deprecated].

[gh]: https://github.com/jingweno/gh
[hub]: https://github.com/github/hub
[deprecated]: github/hub#475

Matches thoughtbot/laptop:

https://github.com/thoughtbot/laptop/commit/d9a9dfe09bf5898172c752c87be7c5ae6537c5f2
Remove unused plugins after rcup

Running `rcup` should remove any plugins that are no longer in use. For
instance, we recently replaced `rename.vim` with `eunuch`. `rcup`
installs the new plugin but does not clean up `rename` from the plugins
directory. The addition of `PluginClean!` does this (without
confirmation).
Fix the zsh config glob

The glob used a modifier, but it was modifying a directory instead of a
glob that matched all the files in the directory. Add the missing `*` to
fix this.
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`.