Allow customizing at the point of inclusion

[damithc]

Suggestied feature: the following code will include foo.md as per normal but will replace all spans/divs with id title in foo.md with New Title and level with 2:

<include src=”foo.md”>
 <span id=”title”>New Title</span>
 <span id=”level”>2</span>
</include>

foo.md:

## <span id="title">Dummy Title</span>

This is a level <span id="level"/> topic.

The level of this topic is <span id="level"/>.

Rationale: this will greatly increase code reuse as it allows authors to write templates and reuse them while customizing them at the point of reuse (somewhat similar to Jekyll tempates https://jekyllrb.com/docs/layouts/).

Points to consider: What about variations that cannot be captured in spans e.g., html attribute values? For example, if we want to use the following code fragment as a template but we want the background color to be confirguragle at the point of reuse

 <box background-color="white" border-color="#cccccc"><md><span id="content"/></md></box>

[damithc]

@jamos-tay this might go well with your work as this will be a non-trivial feature related to author-usability.

[jamos-tay]

Hmm, this looks pretty similar to markbind variable functionality (_markbind/variables.md). What if we allowed the user to specify markbind variables that are only used in that include's context, so like:

<include src=”foo.md”>
 <span id="title"><span>New Title</span></span> // same syntax as variables.md
 <span id="level">2</span>
 <span id="background-color">white</span>
 <span id="border-color">#cccccc</span>
</include>

foo.md

## {{ title }}

This is a level {{ level }} topic.

<box background-color="{{ background-color }}" border-color="{{ border-color }}"><md><span id="content"/></md></box>

Naturally include-specific variables take precedence over variables.md.

One issue is if we want default values for variables. Can we extend the variable functionality to allow for these, e.g. {{ background-color | default #ffffff }}? I can see it having a use even outside include.

[damithc]

@jamos-tay note that we use nunjucks for implementing variables and I suspect nunjucks does a pass through the code first before markbind gets a chance to process includes, which means all {{ ... }} will be wiped out as nunjucks replaces unknown variables with empty string. If we want to use variables, we might have to come up with our own syntax e.g., {< ... >}. I'm just guessing though.

[jamos-tay]

Hmm, I was thinking we could just inject variables, I guess I'll look into it and if necessary we can use our own syntax