Support specifying include variables inline

[jamos-tay]

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

• [X] Enhancement to an existing feature

Fixes #642

What is the rationale for this request?

Provide a simpler syntax for specifying include variables

What changes did you make? (Give an overview)

Authors can include variables inline by adding var-... attributes:

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

Used var instead of data as it's more variable related

[yamgent]

Just one query, the rest looks good.

[jamos-tay]

Rebased, see comment

[damithc]

@jamos-tay there was a suggestion from @acjh to use data- as the prefix (apparently, that's what HTML standard recommends). Was it changed to var- later?

[acjh]

Since the <include> tag is not in the rendered HTML, var- attributes are acceptable.

[jamos-tay]

Yeah, I figured var- might be better since it's more related to variables. data might collide with some other libraries.