[feature request] "anchors of Headers" support for markdown

[HaveF]

Dear iDL,

I wanted to take a moment to express my appreciation for the work you have done. I'm big fan of your vega language. I have been using it for several years and I am consistently impressed with the quality and functionality of it.

Several days ago, I want to find something to build an interactive page. After a lucky search, I find this. It is what I want. It makes to build interactive web pages much easier.

I was wondering if it would be possible to add a feature to support "anchors of Headers" feature. This feature is not quite common in paper, but it is very common in web. If the anchor exist, I can add [create an anchor](#anchors-in-markdown) to denote the specific section. User can create ToC(table of content) by hand, or create a link some where to point that section.

This would make it much easier for users to quickly browse and navigate through the content. I know this would take some extra effort, but I believe it would be a valuable addition to the project.

Thank you again for all of your hard work and dedication to this project.

[HaveF]

I found id of header could work as expected.