Change default to not highlight automatically

[wooorm]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)

Problem

GH, and most places, don’t highlight code automatically. They instead need a special language flag.
This project does attempt to highlight automatically.

Solution

• change the default to stop automatically highlighting
• Add a new option to turn it back on (changing the overloaded meaning of subset)

Alternatives

n/a