Added the ability to configure the start command via a config file.

This allows you to place a ppm.json file within your project root and then simply issue `ppm start` to start the server.

ppm.json file:
```
{
    "bridge": "symfony",
    "port": 8888,
    "workers": 8,
    "app-env": "dev"
}
```

I'm not 100% convinced that json should be the format used. If you have any preferences let me know.