Improve accordion styling options

[charlie-yao]

Resolves #150

• The header, button, and panel elements can now be styled in multiple ways:
  • className prop as a function that receives the accordion's state information
  • className prop as a string
  • Default CSS class that can be targeted
  • style prop as a function that receives the accordion's state information
  • style prop as an object
• The expanded and disabled states are now reflected on the header and panel elements as HTML data attributes, e.g. data-expanded="true"
• React ARIA widgets no longer provides default styling to show the expanded/collapsed state, i.e. removes the react-aria-widgets-hidden class