Code blocks: Implement inline markdown support for heading

[nbriannl]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [ ] Documentation update
• [ ] Bug fix
• [ ] New feature
• [X] Enhancement to an existing feature
• [ ] Other, please explain:

Fixes #1119

What is the rationale for this request?

It would be nice to support inline markdown formatting as we support it in other places.

What changes did you make? (Give an overview)

Render heading markdown-it-attr as inline markdown using markdownIt.renderInline(heading)

If heading contains rendered inline markdown, increase line height because default line-height will make the headings crowded. As seen below

with line-height: 1.5

with lint-height: 1

Added tests as well.

Provide some example code that this change will affect:

```html {heading="### Heading level 3"}
<foo></foo>
```

```html {heading="Line breaks<br>Are possible<br>[And links!](https://markbind.org)"}
<foo></foo>
```

```html {heading="**Bold**, _Italic_, ___Bold and Italic___, ~~Strike through~~, :+1: :exclamation: :x: :construction:"}
<foo></foo>
```

```html {heading="****Super Bold****, ++Underline++, ==Highlight==, %%Dim%%"}
<foo></foo>
```

```html {heading="<blockquote>Using blockquote tag</blockquote>"}
<foo></foo>
```

```html {heading="![](https://markbind.org/images/logo-lightbackground.png)"}
<foo></foo>
```

```html {heading="@[youtube](http://www.youtube.com/watch?v=v40b3ExbM0c)"}
<foo></foo>
```

- Can't support `inline code` text style (because it would break markdown-it attrs)
- Dim does not really work.
- Can't do headers, lists, paragraphs, footnotes, embeds

Is there anything you'd like reviewers to focus on?

Testing instructions:

Try the test code on your own or check out
https://nbriannl.github.io/markbind-plain-site/

Proposed commit message: (wrap lines at 72 characters)

Code blocks: Implement inline markdown support for heading

[openorclose]

How about some tests, one for a normal heading and one markdown heading?

[ang-zeyu]

LGTM 👍, just two nits:

[ang-zeyu]

with some exceptions
Could only mention the inline exclusions ( inline code / dim )

[ang-zeyu]

indent here should be 2 to the left, is github displaying it wrongly?

[nbriannl]

No you're right. The pains of not having eslint consider this file. 😞

[marvinchin]

Just wondering - what if the user wants to use symbols that are part of markdown syntax in the heading? (i.e. user wants *Title* rather than Title)

I believe markdown-it should allow escaping with \. Could we confirm that, and perhaps add that to the documentation?

[damithc]

Just wondering - what if the user wants to use symbols that are part of markdown syntax in the heading? (i.e. user wants Title rather than Title)

In fact, that scenario is not limited to headings, but applies to anywhere markdown syntax is used, right?

[marvinchin]

In fact, that scenario is not limited to headings, but applies to anywhere markdown syntax is used, right?

Yup, I think that should be the case.

I did some digging around and it seems like we already have a page on escaping markdown - my bad for not noticing it before:
https://markbind.org/userGuide/tipsAndTricks.html#tip-escaping-characters

I think my main concern here would be whether or not introducing this change would lead to "breaking" existing users' code block headings which use characters from the markdown syntax. I don't imagine that there are many existing usages that would fall under this category though, since this is a fairly new feature. Do you think it's necessary for us to mark this as a breaking change?

[marvinchin]

Found the documentation that I was requesting 👀 LGTM!

[nbriannl]

Noting conflicts. I'll resolve it asap.

[nbriannl]

Rebased on master, ready for merge