Add autoenv, automatic environment setup

Using a hook, automatically detect when `.enter` and `.exit` scripts
and source them when changing the shells current directory.

Fixes #3.

zshrc

@@ -12,6 +12,7 @@ source-plugin() {
 source-plugin zsh-autosuggestions
 source-plugin zsh-history-substring-search
 source-plugin zsh-syntax-highlighting
+source-plugin autoenv
 
 # Disable non end-of-line autosuggest accept widgets
 ZSH_AUTOSUGGEST_ACCEPT_WIDGETS=(end-of-line vi-end-of-line)