feature: support line number in code blocks #178

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Feature request

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What problem does this feature solve?

Show line numbers in code blocks.

What does the proposed API look like?

Globally enabled:

markdown:
  lineNumbers: true

Toggle line numbers per code block:

```js {lineNumbers:true}
console.log(42)
```

How should this be implemented in your opinion?

Implement it in the markdown/highlight-plugin.js

Are you willing to work on this yourself?

If no one else will.