Support multiple Markdown files

[rchrd]

It would be useful to use multiple Markdown files instead of only slides.md.

The slides.md file can become quite large and unclear. And it is also not easy to work with multiple people on a presentation.

The easiest would be to combine all .md files in the workdir and serve it as slides.md to Reveal. Sorting can then determine the order of the files (01-title.md, 02-intro.md, etc).