Add zshenv file

From the [Zsh manual](http://zsh.sourceforge.net/Intro/intro_3.html):

> '.zshenv' is sourced on all invocations of the shell, unless the -f option is
> set. It should contain commands to set the command search path, plus other
> important environment variables. `.zshenv' should not contain commands that
> produce output or assume the shell is attached to a tty.

Why is this important? [Alfred](http://www.alfredapp.com/) workflows run in
non-interactive shells. When the `$PATH` is set, or `rbenv` is initialized, in
`zshrc` instead of `zshenv`, those workflows will not use the correct Ruby
version and might not have access to certain bin files, such as those from
`$HOME/.bin/` or Homebrew.
Remove old doc directory

Resolves #276
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).