Include parts of other Org files in a zpresentation

from https://www.reddit.com/r/emacs/comments/702577/i_just_released_an_early_version_of_zpresent_a/dn046vb/?context=3 .

There are two ways for this to work:

1. including other files quoted in the presentation (e.g., "quote this snippet of source from my init file in this presentation")

2. including other files as components of the presentation (e.g., "for the next set of slides, look at this file")