Support syntax highlighting for additional languages

[thesamet]

I'm trying to add an additional language (protobuf) to the syntax highlighter.

Currently, microsites supports only scala, java and bash. This is hard-coded in two places:

1. highlight.pack.js is pre-built with support only for these languages.
2. https://github.com/47deg/sbt-microsites/blob/a7b51de6643a5b14c325e1b7f7b176a64d1dcf99/src/main/scala/microsites/layouts/Layout.scala#L213-L217

One way to go about (1) is to allow users to bring their own highlight.pack.js. Maybe it is possible now? I couldn't find any documentation on this. Another way about it is to create a setting that would <script src> additional languages from https://cdnjs.com/libraries/highlight.js/ . This can not be accomplished today with CdnDirectives(jsList=...) since those scripts are sourced after hljs.configure()` gets called.

For (2), we can add an SBT key for micrositeHighlighterLanguages that defaults to Seq("scala", "java", "bash") for backwards compatibility. The same key should probably control the additional languages to download from the CDN.

Thoughts? Happy to send over a PR.

[juanpedromoreno]

Thanks @thesamet , both options look good. Perhaps option 2 is more SBT/Scala friendly, in my opinion... And yes! any contribution is totally welcome :)