Add line highlighting for live code blocks

[hectahertz]

In order to emphasize a particular range of lines, this adds the highlight attribute on the live code blocks. The line highlighting will disappear when the live example is edited.

This implementation is quite simple, and we could improve on this to support highlighting multiple ranges in the future.

The approach is a bit unorthodox, as react-live does not support line highlighting yet.

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Latest commit: 48b731b

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[siddharthkp]

This is beautiful, extra points for handling pristine/dirty state!