Variables in includes: provide a simpler syntax

[damithc]

Current: syntax for specifying include-variables:

<include src="foo.md">
  <span id="x">bat</span>
</include>

Suggested: also allow this syntax:

<include src="foo.md" x="bat" />

Not sure how hard it is though.

[acjh]

We should prefix variables (along the lines of data-*) to not mix with attributes.

[damithc]

We should prefix variables (along the lines of data-*) to not mix with attributes.

Good idea 👍

[yamgent]

Shouldn't be too hard to implement, we can use Cheerio to check the name of each attribute and process them if they have the data- prefix.

Of course, the content that these variables can have would be quite limited compared to the current way (e.g. you cannot put HTML code with the new syntax), but since we are supporting both version that shouldn't pose a problem.

[jamos-tay]

Sure, I'll wait for #627 to be merged first though cause there'll be conflicts