feat: add onSync callback to resolve #87

[haoliangwu]

the new behavior

add onSync callback to ScrollBar comp.

why

the reason is that I think it is better to decouple the implementation of how to sync perfect-scroll container by invoking ps.update() with DI pattern. In this way, we don't need to expose several props which stand for some special meaning, eg: debounceUpdate, debounceTime, debounceMaxTime and so on. We pass ps instance to user and let them decide how to sync the container(even not by involing ps.update()).

how to use it

please refer to example.js

[goldenyz]

LGTM, please update as the comments.
It is really kind of you if update the README.md also