Add option to save toggle state

[rhwd]

On the Reveal class:

• Create new function to save the state of a certain item on the session storage.
• Save function is always called on toggle()
• If data-reveal-store-id is empty, return null
• On connect() call restoreState()
• On restoreState() if there's no save state, return null

On the Landing page

• Create new example button
• Create a grid to center all examples

If you have any suggestions, please tell me!

[rhwd]

What do you think about it @michaelbaudino, @guillaumebriday?