Added nvm.zsh to detect current Node.js version

lib/nvm.zsh

+# get the node.js version
+function nvm_prompt_info() {
+  [ -f $HOME/.nvm/nvm.sh ] || return
+  local nvm_prompt
+  nvm_prompt=$(node -v 2>/dev/null)
+  [[ "${nvm_prompt}x" == "x" ]] && return
+  nvm_prompt=${nvm_prompt:1}
+  echo "${ZSH_THEME_NVM_PROMPT_PREFIX}${nvm_prompt}${ZSH_THEME_NVM_PROMPT_SUFFIX}"
+}