Issue title: Adding balanced text layout would really improve the slides
 Issue body: https://github.com/adobe-webplatform/balance-text implements this as a polyfill, pending a new CSS property, but balancing the text in general would improve layout of bulleted lists, headers and anything other than huge narrative paragraphs

 Comments: 
 Comment 0: Didn't know that existed, thanks for sharing. Unfortunately it's not something I'm interested in adding to reveal.js since it requires jQuery. Also, going through all text using JS scares me in terms of perf, particularly for big presentations.

 Comment 1: It may be worth adding text-wrap:balance to the style sheets, pending browser implementations of this proposal.

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