Configuring scrollOffset per step

[weizhongtan]

Firstly, just want to say thank you for developing this library! It's been really fun to work with and relatively easy to configure for various use cases.

One use case I wasn't able to configure was setting a custom scrollOffset for a specific step. This is particularly useful for mobile, where we want to scroll different amounts depending on the height of the target.

Is this something that would be viable to add? I am happy to raise a PR if so.

Thanks!