Custom CSS Classes Support

[giuliano-abinbev]

The library currently supports a custom css class that goes along with range-slider in the root/wrapper element. This issue is more about allowing developer to completely replace all range-sliders classes with custom ones.

Motivation

Allowing developers to completely replaces the default css variables allows them to:

• Support CSS Modules
• Have more granular control
• Avoid collisions (e.g. an app could already have a global range-slider class for something else)

Suggestion

Replace the className props with a classNames prop that accepts an object

type Props = {
   classNames?: {
      slider?: string,
      thumb?: string,
      range?: string,
   }
}

classNames should be optional as well as the keys, and default to the current classes.