Add an autocommand to re-add highlight groups

[jez]

Fixes #211
Fixes #376

The relevant motivation and summary are in the comment that I've left behind in
the file.

I will mention that this technique is not uncommon:

https://github.com/search?q=%2Fautocmd+ColorScheme%2F&type=code

A similar pattern comes up in most other syntax files that define custom
highlighting groups.

Before

before.mov

After

after.mov

[jez]

Friendly bump on this! @alerque or @fmoralesc I'd love if one of you two had a moment to take a look at this. Happy to make changes as needed.

Thanks in advance.

[alerque]

Sorry it took so long for this to hit my radar at a time when I had time to actually read it. It's an obvious win. Thanks for the contribution.

[jez]

No worries, thanks for the review and Happy Thanksgiving 🙏