Improve A11y support

[cassiozen]

Hi, I want to help with a PR for a11y - here are the things I would like to tackle:

• Change getHandleProps to return by default: role ('slider'), aria-valuemin, aria-valuemax and aria-valuenow.
• Change getHandleProps to not set tabindex and not set outline to none. Ideally, update the examples to use Buttons for handles (since they already are interactive elements and have a tab stop set by default).
• Add support for keyboard navigation (When a handle is focused, the user should be able to use left/right arrows to change value)

Do you think these changes make sense for the library?

[tannerlinsley]

Absolutely! Go for it :)

[cassiozen]

PR: #17