Set Input\Cli default filter to "string"

In CLI mode, if we type:

```
cli.php -a "foo bar"
```

The `$input->get('a')` will return `foobar`, but we may hope it be `foo bar`.

I think we can set default filter to `string` instead `cmd` in CLI Input, because I guess that there won't have security issue with input value if someone using CLI to operate system.