Parse Procfile command with environment variables

Currently Procfile commands only get parsed with environment variables for the commands arguments. For example:

    .env
    WEB=node app.js

    Procfile
    web: $WEB

Doesn't work. This is an edge case, but useful if you want to run `node app.js` in production and `supervisor app.js` in development.

I've changed the parsing to also check the main command, and not just the arguments.