Add support for extra highlight.js languages

[bkbooth]

This adds support for extra highlight.js languages by assigning an array to either config.toml or a page/post's front matter:

config.toml

# ...
[params]
  # ...
  hjsStyle = "default"
  hjsExtraLanguages = ["elm"]

content/post/my-blog-post-about-elm.md

+++
title = "My Blog Post About Elm"
# ...
hjsExtraLanguages = ["elm"]
+++

...

I've also upgraded highlight.js from v9.6.0 to v9.8.0

[vjeantet]

Thanks !