Optionally use environment variables with a database URL to configure mi...

...grations

The following config/config.json would work with heroku:

```json
{
	"development": {
		"username":"username",
		"password":"password",
		"database":"db",
		"host":"127.0.0.1",
		"dialect":"postgres",
		"protocol":"postgres"
	},
	"production": {
		"use_env_variable": "DATABASE_URL",
	}
}
```

Provided you've set up the database and configured ```NODE_ENV=production``` on heroku.