Proposal: Lock versions to Highlight JS

Highlight JS [has some new maintainers](https://github.com/highlightjs/highlight.js/issues/1678) after a period of slow development and can be expected to be going through some more rapid changes in the coming weeks and months. I expect that users of this lib (myself included) will have requests to bump the highlightJS dependency to get the new goods.

I was think that, given this is largely just a wrapper library, that is might make sense to mirror the major/minor release numbers for an easy reference. This would, of course, make it awkward to release additional features to the wrapper without waiting and bundling with highlight releases, but if the maintainers feel this library is essentially done outside of future hightlightJS it would probably be fine.