allow custom filename not only for download, but also for show function

Why not allow a custom filename for the show function? I know, basically it's just
```
PDF::load($view, 'A4', 'landscape')->download('filename', array('compress' => 1, 'Attachment' => 0));
```
vs
```
PDF::load($view, 'A4', 'landscape')->show('filename');
```
but I think for the sake of readability this should be added.