Feature Suggestion: Create one slideshow from multiple documents

[bradparks]

Hey! I love the document mode idea, as it lets me write something as a document that can be browsed in our own github repo, and it can be converted to a slideshow as well.

I think it'd be super awesome if I could somehow pass in multiple documents, which would get appended together into a single huge slideshow. This would let me continue to write separate markdown documents for my app, but break it down naturally.

e.g. If I had the following Markdown files, it'd do the same as you do with document mode, but it'd combine the following files in order to create a single slideshow.

intro.md
app_startup.md
configuration.md
support.md

so running something like:

markdown-to-slides -d intro.md app_startup.md configuration.md support.md > slideshow.html

would generate one slideshow of all the above markdown files.