Does this work with PNG?

Does this lib. work with PNG?

```typescript
let img1 = `data:image/png;base64,${fs.readFileSync(
    './foo.png',
    'base64'
)}`;
let img2 = `data:image/png;base64,${fs.readFileSync(
    './expected_foo.png',
    'base64'
)}`;

resemble(img1).compareTo(img2).onComplete(data => {
    console.log(data);
});
```

**{ error: 'Error: Unsupported image type' }**