Add theme option for languages with syntax highlighting in code blocks

[ayhon]

I would like to request a theme option, like navigation or externalLink, that would take a list of names of programming languages whose syntax highlighting are supported.

Key	Default Value	Description
highlightedLanguages	["javascript","react","typescript"]	Names of programming languages which get highlighted in code blocks

I've been trying to set up syntax highlighting for languages other than javascript/react in my blog. As I figured this wasn't an interest exclusive to me, I thought I'd search how to add more languages to the highlighting in the GitHub repo and came across issue #563.

I'm not really used to gatsby or react yet, so I don't fill confident implementing this feature on my own, or shadowing the theme to hack it in for my blog. I managed to get haskell syntax highlighting working by following the instructions at #563, but I can't add any other language to highlight after that, so it doesn't really cover my use case.

Also, I believe this feature can be interesting enough to warrant implementation on the theme itself.

Thank you for your time.