fix(config): default port and custom ports

closes #519

- `ghost install` -> uses the next available port (starts with 2368)
- `ghost install --url http://localhost:4000 (uses 4000)
- `ghost install --url http://localhost:4000 --port 2390 (uses 2390)

Author: kirrg001

lib/commands/config/advanced.js

@@ -55,7 +55,6 @@ module.exports = {
             const port = parseInt(url.parse(config.get('url')).port || BASE_PORT);
             return portfinder.getPortPromise({port: port});
         },
-        default: 2368,
         type: 'number',
         group: 'Ghost Options:'
     },