Add prop to specify back button behavior

[lindboe]

Adds a prop that allows configuration of back button behavior. By default, it's still a no-op, but also allows either navigation to the previous step, or stopping the walkthrough all together.

This was especially helpful in my case, because the tutorial as designed did not use buttons (clicking the mask or the tooltip would advance the tutorial), so it's helpful for the back button to still behave as the user expects.

Part of a series of PRs for more flexible styling:

• Add nextOnOutsideClick option #238
• Make Tooltip position-aware #239

[naoyamakino]

+1

[mohebifar]

Hi @lindboe! Sorry for the overdue response. I see you've had a lot of valuable PRs. I recently refactored the code base and migrated it to TS and functional components. I was wondering if you are still willing to update and merge your PRs.

[lindboe]

Hi @mohebifar! I'm not currently using this library so it's not very likely I'll get around to this soon

[mohebifar]

No worries. I can integrate these into the new code.