[feature request] Custom Heading IDs

[ottojo]

It would be great if custom heading IDs as described here: https://www.markdownguide.org/extended-syntax/#heading-ids would be supported in codimd.
This would allow making intra-document links with predictable and stable identifiers.

## Chapter 2: Examples for Links {#links-examples}
...
## Chapter 3
Reference to [links](#links-examples)