Allow custom CSS class prefixes (avoid style collisions)

All classes (shepherd-feed, shepherd-tag, shepherd-ad, active) are hard‑coded. If a site already uses those names, styles may clash. Add an optional classPrefix option (default "shepherd-") that prefixes every generated class. Update the CSS file to use the variable prefix (e.g., .{{prefix}}feed). Provide a small build‑time script or runtime string replacement.