Add the global `node_modules` path to `NODE_PATH`

In Ruby or Python for example, we can `require` gems or `import` packages which have been installed globally. Even with rbenv/pyenv, no config required.

In Node however, we have to add manually the global `node_modules` path to `NODE_PATH`. It needs to be changed frequently in ndenv environment.

With this commit the global `node_modules` each version has will be added automatically just before execution in `ndenv-exec`.