Use `npm start` if Procfile is absent

This recent feedback from customer @aseemk echoes my sentiment:

> Procfile: you guys require this currently, which makes sense when you want to have multiple processes or tweak the command, etc. But I'm building an open-source project w/folks that we want to work across multiple PaaSes, and most other Node.js PaaSes support just defaulting to the common Node convention of `npm start`. It'd be nice if you guys defaulted to this also, if a Procfile wasn't present, for the simple/typical case of just one web process. (This'd lift the requirement for a Procfile for Node.js apps, just like is the case on Heroku for PHP apps.)