[Feature Request] Load multiple source files in from `sourceUrl`

[sauln]

Currently, it is possible to load markdown slides from a separate file like

var slideshow = remark.create({
    sourceUrl: 'slides.md'
});

It would be nice to accommodate a list of files. Maybe something like:

var slideshow = remark.create({
    sourceUrls: ['introduction.md', 'body.md', 'tangent.md', 'closing.md']
});

This would help in the organization of large slide decks and in reusability of slide decks.

[valvin1]

I'm not an experienced javascript developper but I think I did something which works. I'll PR it.