Add slide state

[rawrmonstar]

Description

This adds a state prop to <Slide /> that accepts a string. By default the behavior is to look at the currently rendered <Slide /> and add the state prop to the document root (html tag) as a class. This behavior can be modified by passing a onStateChange function prop to the <Deck />. The function excepts 2 arguments: (previousState, nextState) => { /* do whatever you want */ } and is called when a slide with a different state prop than the previous one is mounted--this includes from when a slide has a state and the next slide does not have a state.

Fixes #434

Type of Change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update (included in this PR)

How Has This Been Tested?

Tested manually using spectacle-boilerplate and I've added some tests to our suite.

Checklist: (Feel free to delete this section upon completion)

• I have updated type definitions in DefinitelyTyped for any breaking API changes. PR: <Spectacle - Slide and Deck component prop updates DefinitelyTyped/DefinitelyTyped#31599>