Extend Present syntax with .include directive

[cmars]

Would be nice to be able to include snippets of Present markup from other files.

.include file.article

would insert the contents of an article file fragment at that point in the document.

[amitu]

This should be possible, https://godoc.org/code.google.com/p/go.tools/present#Register, but I suppose it should go to present. I do not want present and represent to have different file formats.

[amitu]

Also there would be whole issues with which slide numbers, which I suppose can be handled by massing slide number ranges as part of .include parameter, eg:

.include old_talk.slide 20..24