Prevent launching node.js script file in working directory.

The PATHEXT environment variable on Windows often includes .JS, which means running "node" can try to execute a file named "node.js" in the current working directory. As npm-installed executable scripts are often executed in directories containing .js files, this is not unlikely.

This change works around it by locally stripping the .JS extension from the PATHEXT environment variable.
Merge pull request #5 from bwrrp/js-pathext-workaround

Prevent launching node.js script file in working directory.