add first pass at adding NVM to modules

modules/nvm/README.md

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+NVM
+====
+
+Configures [Node][1] local gem installation, loads version managers, and defines
+aliases.
+
+NVM
+---
+
+[The Node Version Manager (NVM)][2], allows for managing multiple, isolated
+Node installations and node_modules sets in the home directory.
+
+#### Auto-Switch
+
+To enable auto switching the Node version on directory change based on the
+.nvmrc file, add the following line to *zpreztorc*:
+
+    zstyle ':prezto:module:node:nvm' auto-switch 'yes'
+
+
+Authors
+-------
+
+*The authors of this module should be contacted via the [issue tracker][5].*
+
+  - [Jim Ray](https://github.com/jimiray)
+
+[1]: https://nodejs.org/en/
+[2]: https://github.com/creationix/nvm

modules/nvm/init.zsh

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+### NVM
+export NVM_DIR="$HOME/.nvm"
+. "/usr/local/opt/nvm/nvm.sh"
+
+autoload -U add-zsh-hook
+
+add-zsh-hook chpwd load-nvmrc
+load-nvmrc
+
+load-nvmrc() {
+  local node_version="$(nvm version)"
+  local nvmrc_path="$(nvm_find_nvmrc)"
+  if [ -n "$nvmrc_path" ]; then
+    local nvmrc_node_version=$(nvm version "$(cat "${nvmrc_path}")")
+    if [ "$nvmrc_node_version" != "N/A" ] && [ "$nvmrc_node_version" != "$node_version" ]; then
+      nvm use
+    fi
+  elif [ "$node_version" != "$(nvm version default)" ]; then
+    echo "Reverting to nvm default version"
+    nvm use default
+  fi
+}