Conditional spriting based on image url

Add an option to conditionally filter images based on the image URL

For example, any image with `?__spriter` will be sprited

```
background-image: url('./images/test.png?__spriter');
```

No files or tests are named. Start by locating the image URL filtering and sprite-generation entry points, then trace how CSS background-image URLs are parsed. Done means URLs containing the `?__spriter` marker are selected for spriting while other images remain unchanged, with coverage for both cases.

Written by the indexing model from the issue text.