Adds options for update check

If the env variable UPDATE_CHECK is set to "daily", "weekly" or
"monthly", there will be a message if you're late on your update.

If the env variable UPDATE_CHECK_ALWAYS is set to "true", you will get a
message telling you that your system is up to date.

zshrc

@@ -10,6 +10,10 @@ ZSH_CUSTOM=$HOME/.config/dotfiles/zsh/oh-my-zsh
 
 ZSH_THEME="laptop"
 
+if [ -f $DOTFILES/zsh/env.zsh ]; then
+    source $DOTFILES/zsh/env.zsh
+fi
+
 source $DOTFILES/zsh/config.zsh
 source $DOTFILES/zsh/path.zsh
 source $DOTFILES/zsh/exports.zsh