Allow accessing page variables from outside

[damithc]

e.g., allow <include src="foo.md#title"/> (where title is a page variable in foo.md)

Some dicussion about this feature can be found here

[jamos-tay]

Follow up from #735

Okay, I'm having trouble with this too =P

We render the entire page's variables before any includes are processed, which means we can't read variables from inner files before hand. I probably have to read them separately (which means having my own include logic, not ideal), or work the page logic around so variables are rendered after includes (i.e. the preprocess step).

I think this is because the definition of a variable changed from 'global constant' to just 'a variable'. This affects the way they should be handled, so I think a rewrite might be in order.

[damithc]

yes, this one will be extremely challenging :-p
would something like this work? treat variables like global constants but internally prefix each with the path to the page e.g., docs.userGuide.gettingStarted.title. That way, they can be made visible everywhere but there is no danger of clashes.
Just a thought.

[acjh]

Further discussion about this feature can be found here: #674 (comment)