Add Ability to specify how deep page headers should go

[bretterer]

Feature request

It would be nice to have the ability to specify how deep the extractHeaders method goes.
Currently, it has a hard coded value of ['h2', 'h3']
Our current markdown files use h4 as well in order to handle some table of contents for the page in sidebar form.

What problem does this feature solve?

When doing a table of contents in the sidebar, this feature would allow you to specify how deep the TOC should go based on headers.

What does the proposed API look like?

config.js

module.exports = {
    themeConfig: {
        parseHeaders: ['h2','h3','h4']
    }
}

How should this be implemented in your opinion?

The above would be a themeConfig option and will replace the default hard coded value located at
https://github.com/vuejs/vuepress/blob/master/packages/%40vuepress/core/lib/prepare/Page.js#L92

Are you willing to work on this yourself?**

Yes

[Leedehai]

This is a much needed feature.
Additionally, I wish the sidebar is able to handle "skipped" headings, i.e. a h3-level heading directly under h1 without h2 in between, and a h4-level heading directly under h2 without h3 in between. Like so:

h1
|-- h3 (this)
|-- h2
|   |-- h4 (and this)
|   |-- h3
|   |    |-- h4
|   |    `-- h4
|   `-- h3
`-- h2
    |-- h3
    `-- h3

[bretterer]

Looking into how to code this, Once issue is it seems that currently, the Page.js does not have access to the config file. I have been looking for samples of the prepare files requiring the config file and and falling short. It may require some re-structure/re-work of how that is set

[0w0k]

Welcome to pull requests, This is a much needed feature.

[Leedehai]

Heck, even Vuepress's own README contains skipped headings: an h2 heading ("Status") is not governed by any h1 heading. The first h1 heading in the article ("Vuepress") is below it. :)

[bretterer]

Picking this up this week. Targeting to have a PR by the end of week

[yiwiz-sai]

this is obviously necessary.

[bretterer]

@pianoboysai I believe this was merged in through another PR #1945 after being told to "avoid wasting your time" and let the "feature be implemented by official collaborators"