Feature Request: Add offset prop for autoplay initial delay

### Summary
Add an `offset` prop to delay the first autoplay transition.

### Problem
When using multiple sliders with autoplay on the same page, they all transition at the same time. There's no way to stagger them without manually controlling each slider with custom code.

### Proposed Solution
```javascript
const settings = {
autoplay: true,
autoplaySpeed: 5000,
offset: 2000, // Delay first transition by 2 seconds
// ... other settings
};
```

**Behavior:**
- Without offset: 0s → 5s → 10s → 15s
- With offset (2000): 2s → 7s → 12s → 17s

The offset only delays when autoplay starts, not the interval between slides.

### Contribution
I'm happy to submit a PR if this sounds good. Let me know!