Make TeX a first class compilation target

[breck7]

TeX (and its dialects), as far as I can tell can't be beat when it comes to generating beautifully typeset documents for printing.

So it makes sense to have out of the box compilation of Scroll articles to PDFs (via a TeX intermediate step), in addition to HTML.

Branch in progress: https://github.com/breck7/scroll/tree/tex