Feature request: present subtree, narrowed buffer, or region

[ksafford]

Consider an org file that is structured like:

* Presentations
** Presentation1
*** First slide
- item 
- item
*** Second slide
- item
- item
** Presentation2
*** A different first slide
- another item 
- another item
*** A different second slide
- item
- item

It would be great to be able to select one of Presentation1 or Presentation2 in a region, or narrow to one of those subtrees and call org-present-region or org-present-narrowed-buffer or to simply set the point on a heading for a subtree and call org-present-subtree and have a presentation open as though the headings below Presentation1 were top-level headings in a separate file.

With some guidance, I would be happy to attempt to add this feature.