nodenv plugin

The primary job of this plugin is to provide nodenv_prompt_info which can be added to your theme to include Node version information into your prompt.

To use it, add nodenv to the plugins array in your zshrc file:

plugins=(... nodenv)

Functions

nodenv_prompt_info: displays the Node version in use by nodenv; or the global Node version, if nodenv wasn't found. You can use this function in your prompt by adding $(nodenv_prompt_info) to PROMPT or RPROMPT:
```
RPROMPT='$(nodenv_prompt_info)'
```