Improve debugging info

[wbyoung]

Include the following in the output of __avn_debug automatically:

• [ ] The current version of avn, node, and npm
• [ ] All installed plugins & allow them to produce debug info which should include
  • [ ] The version of the plugin
  • [ ] The version of the underlying tool via something like:
    nvm --version 2> /dev/null || echo "missing"
  • [ ] For nvm specifically, it should determine if it was installed improperly via homebrew
• [ ] The result of env (or an abbreviated list)
• [ ] The current shell
• [ ] The presence of other tools like rvm or autoenv in any profile files
• [ ] Details about the current OS