made shell tasks execute npm scripts for better cross platform support

I couldn't get this project to work on Windows at first, and didn't want to mess around with bash profiles and environment variables. Thankfully, there was a much cleaner solution that involves a feature of `npm`: scripts.

When node modules install, a `.bin` folder is created and any module binaries are placed in it, and the `"scripts"` section of `package.json` can reference these binaries just by their name.