[Perf] Don't load all languages in the highlight.js plugin

[jsulpis]

Hi,
I noticed that the highlight.js plugin loads all languages by default:

reveal.js/plugin/highlight/plugin.js

Line 1 in 4725350

import hljs from 'highlight.js';

The full script is huge, 287.4kB minzipped.

The highlight.js doc suggests to include only the languages needed instead. Maybe with the require syntax the plugin could take an optional parameter to include only specific languages ?

This would be a huge performance gain for all users of the plugin.