Add option for highlight plugin to register additional languages via callback

[rajgoel]

This PR adds an optional parameter callback to the configuration of the highlight plugin. If a callback function is provided, the function is called with hljs as argument before the plugin starts to highlight the code.

Sample configuration

Reveal.initialize({
  // ...
  highlight: {
    callback: registerAMPL 
  },
  // ...
});

A new language can then be added, by including the respective function as shown below:

function registerAMPL(hljs) {
  hljs.registerLanguage("ampl",function(hljs){
    return   {
      contains: [ 
        hljs.HASH_COMMENT_MODE,
        hljs.C_BLOCK_COMMENT_MODE,
        {
          className: "number",
          begin: String.raw`\b(\d*\.?\d+)\b`,
          relevance:0
        },
        {
          className: "strong",
          begin: String.raw`^ampl[\?:]`,
          relevance:0
        }
      ],
      keywords: { 
        keyword: 'maximize minimize subject to set within union inter diff symdiff cross sum in by mod var param default setof dimen symbolic if then else',
        type: 'binary integer',
        string: 'reset model solve data option solver solver_msg solution_precision display include print printf quit',
        built_in: 'abs max min sqrt round trunc ceil floor',
        number: 'Infinity',
        symbol: '_nvars _varname _var'
      } 
    };
  });
}

Fixes #2761

[hakimel]

This is a good addition but I made two adjustments:

1. Renamed the option to beforeHighlight to more clearly indicate when this is called.
2. Moved the callback out of the forEach loop to avoid triggering once per code block.

Thanks!

[rajgoel]

Thanks a lot for including this 👍

[maxmeyer]

This is very helpful. @rajgoel Is there any place where this is documented?

[hakimel]

I added it to the docs here: https://revealjs.com/code/#the-highlight.js-api-%26-beforehighlight-4.2.0

[Martinomagnifico]

Great addition, Asvin!

[maxmeyer]

@hakimel Great! Thanks.