New method: withPrefix

Receives all input parameters that start with the given prefix.

```php
$input->do("save")
	->withPrefix("io_")
	->call("processIo");

function processIo(InputData $data) {
	// $data["product_id"] // from input name=io_product_id
}
```

```html
<form method="post">
	<input name="io_product_id" />
	<button name="do" value="save">Save</button>
</form>
```