added override option for config file

Currently there is no easy way to have a module which depends on npm_lazy and have it run with a different configuration. 

With this change you can run npm_lazy by doing this:
- create package.json
- run npm install --save-dev
- create custom configuration in someconfig.js
- start npm_lazy with node server --config=someconfig.js

I did this by using the optimist module for easy command line parsing, it might not be to your liking but this could be done without any additional dependencies too.