Accessibility Improvements

[cassiozen]

(Refers to #16)

Improves A11y support:

• Fix: getHandleProps now returns by default: role ('slider'), aria-valuemin, aria-valuemax and aria-valuenow.
• Breaking: Changes getHandleProps behavior to not set tabindex and not set outline to none.
  • Updated the examples to use Buttons for handles (since they already are interactive elements and have a tab stop set by default).
• Feature: Added support for keyboard navigation (When a handle is focused, the user can use left/right arrows to change value)

[cassiozen]

It is up for debate, but you might want to release this as a major version, since it might break existing projects for not providing a tabindex on getHandleProps.