Inject frontmatter properties of a section as DOM properties

[kptdobe]

Once #279 is done, sections will be included by default inside a wrapping element (div or section, being discussed). Leveraging the frontmatter properties of each section and injecting them as DOM properties would be an easy way for a developer to give some styling freedom but also directive to control the section rendering to content authors.

Example:

---
class: my-custom-section-class
data-some-custom-data: my-value
---
section content

would lead to a wrapping DOM element:

<div class="section my-custom-section-class" data-some-custom-data="my-value"><p>section content</p></div>

cc @davidnuescheler

[ramboz]

I'd also propose to let the author specify the tag to be used:

tag: my-tag
class: my-custom-section-class
data-some-custom-data: my-value

to get:

<my-tag class="section my-custom-section-class" data-some-custom-data="my-value"> 
    <p>section content</p>
</my-tag>

The idea is really just to be able to better control semantics and SEO.

The tag would default to whatever the outcome of #279 is (i.e. section or div).

[davidnuescheler]

looking through the PR i think we have gone a little bit beyond the above example provided by @ramboz

i really liked what's outlined and i think as we start thinking of the DOM as becoming the more central payload for the pipeline, i think it is desirable to pass information that we want to use throughout processing along as attributes (or tags) in the DOM, even if they get removed in a final step before the markup is generated.

in that light i think that we could allow non-standard attributes and tags throughout the pipeline as we give the developer the ability to enforce strict compliance at the end of the pipeline if they want to.

[ramboz]

Should be mostly covered by #379 now and in since v4.0.0
@kptdobe Safe to close?