Properly support include tag and --incremental behaviour

[paolobrasolin]

Is there an existing issue for this?

• I have searched the existing issues

Original problem

I'm unable to split pages containing math into multiple parts using import, as aliases get ignored in the included files.

Proposed solution

We definitely need to support the include tag (thus enabling --incremental behaviour too -- see documentation)

Alternatives considered

Right now monkeypatching https://github.com/jekyll/jekyll/blob/1aea1580c193c271b8cc083607105da363af358f/lib/jekyll/tags/include.rb#L174-L177 seems to be the simplest way.

Additional context

This problem emerged trying to support @jonsterling in #19 for his jonsterling/math