Piping doesn't work

Example command
```shell
guzzle print area screenshot | swappy -f -
```

which should work and match
```shell
grimshot save area - | swappy -f -
```


However, swappy fails with 
> unable to load file: /tmp/swappy-stdin-JR2PB3.png - reason: Couldn’t recognize the image file format for file “/tmp/swappy-stdin-JR2PB3.png”