Add option to set title card dimensions

As suggested on the Discord:

Cards are written and created using 3200x1800 pixels, but for users who want to reduce their file sizes this should be adjustable. 

Best implementation would likely be a global `options` option, like so:
```yaml
options:
  card_dimensions: {width}x{height}
```

---

In the web interface this should be in the Settings page alongside card extension.