Create Procfile from `npm start`

A `Procfile` is not required to run a Node.js app on Heroku. If no `Procfile` is present in the root directory of your app during the build process, we will check for a `scripts.start` entry in your `package.json` file. If a start script entry is present, a default Procfile is generated automatically:

```term
web: npm start
```

This is a tested implementation based on ideas and commits from:

https://github.com/heroku/heroku-buildpack-nodejs/pull/49
https://github.com/heroku/heroku-buildpack-nodejs/pull/52