Implement basic support for parsing magic comments

By parsing magic comments, e.g. `% !TEX root = master.tex`, we can implement
for building TeX documents that are split across multiple files.

These magic comments originates from TeXShop, and have fairly wide support
in the most used TeX editors, as well as such LaTeX packages for other editors.

Some additional details surrounding supported keywords, etc., can be
found here: http://pages.uoregon.edu/koch/texshop/version.html