Scrollbars class names

[qodunpob]

In some cases the best way to customize scrollbars is an class names. That's why I'm added 4 new props:

• verticalContainerClassName;
• verticalScrollbarClassName;
• horizontalContainerClassName;
• horizontalScrollbarClassName.

I'm also included classnames package for easier manipulation of class names. And wrote the relevant tests.

[maximkoretskiy]

Duplicate for #62.

[qodunpob]

Sorry, I did not see that pull request. But my decision is different. I propose a separate customization for vertical and horizontal scrollbar. And wrote the relevant tests.

[maximkoretskiy]

Ok. But in fact it does not matter. Repo owner does not watch for PR

[souhe]

Hey, sorry I wasn't watching.
@bashkos could you elaborate more why you think that separating vertical and horizontal classnames is neccessery? What are the usecases? @maximkoretskiy @avanes which solution you prefer? I'm asking because of similar PR #62

[qodunpob]

@souhe
Is not actual for me