Add check-config for php_flag

Added configuration for another ZendDiagnostics check: [PhpFlag](https://github.com/zendframework/ZendDiagnostics#phpflag)

**Example config**

```yml
liip_monitor:
    checks:
        php_flags:
            ignore_user_abort: false
            html_errors: true
            safe_mode: false
```

**Monitor health output**

```bash
$ php app/console monitor:health
OK PHP flag "ignore_user_abort": ignore_user_abort is disabled
FAIL PHP flag "html_errors": html_errors is expected to be enabled
OK PHP flag "safe_mode": safe_mode is disabled
```