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.

README.md

@@ -52,6 +52,7 @@ Put your customizations in dotfiles appended with `.local`:
 * `~/.tmux.conf.local`
 * `~/.vimrc.local`
 * `~/.vimrc.bundles.local`
+* `~/.zshenv.local`
 * `~/.zshrc.local`
 * `~/.zsh/configs/*`
 
@@ -70,6 +71,13 @@ Your `~/.gitconfig.local` might look like this:
       name = Dan Croak
       email = dan@thoughtbot.com
 
+Your `~/.zshenv.local` might look like this:
+
+    # load pyenv if available
+    if which pyenv &>/dev/null ; then
+      eval "$(pyenv init -)"
+    fi
+
 Your `~/.zshrc.local` might look like this:
 
     # recommended by brew doctor

zshenv

@@ -0,0 +1,17 @@
+# use vim as the visual editor
+export VISUAL=vim
+export EDITOR=$VISUAL
+
+# ensure dotfiles bin directory is loaded first
+export PATH="$HOME/.bin:/usr/local/bin:$PATH"
+
+# load rbenv if available
+if which rbenv &>/dev/null ; then
+  eval "$(rbenv init - --no-rehash)"
+fi
+
+# mkdir .git/safe in the root of repositories you trust
+export PATH=".git/safe/../../bin:$PATH"
+
+# Local config
+[[ -f ~/.zshenv.local ]] && source ~/.zshenv.local

zshrc

@@ -57,21 +57,6 @@ bindkey "^Y" accept-and-hold
 bindkey "^N" insert-last-word
 bindkey -s "^T" "^[Isudo ^[A" # "t" for "toughguy"
 
-# use vim as the visual editor
-export VISUAL=vim
-export EDITOR=$VISUAL
-
-# ensure dotfiles bin directory is loaded first
-export PATH="$HOME/.bin:/usr/local/bin:$PATH"
-
-# load rbenv if available
-if which rbenv &>/dev/null ; then
-  eval "$(rbenv init - --no-rehash)"
-fi
-
-# mkdir .git/safe in the root of repositories you trust
-export PATH=".git/safe/../../bin:$PATH"
-
 # aliases
 [[ -f ~/.aliases ]] && source ~/.aliases