NPM package

This PR makes it possible to install passup as a npm package. Still needs testing, but I believe it can work on windows/mac/linux.

passup config is implemented (creates an example config in the user's home directory). passup command works with all command line arguments. 

A few things could be improved. When I install the package locally, it complains about no repository field being present (even though there is one in the package.json file). It's just a warning, so the package still works and installs correctly.