Speaker notes in includes are not published

[paulinevos]

Speaker note commands in included markdown files are not published (and this happens silently). This is not ideal when you decide to split your presentation up sections that are included in the main file.

To reproduce:

demo.md

Some title 
========

<!-- speaker_note: Foo -->
<!-- end_slide -->

<!-- include: included_slide.md -->

included_slide.md

Some other title 
========

<!-- speaker_note: Bar -->

Result: only Foo is included in the speaker notes.

Expected behavior:

Ideally for the included speaker note to be displayed or, if that's complex/not the intended behavior, add it to the speaker notes documentation.

Would be happy to contribute either way!