[ld48] First pass at a simple includes system, because twine sucks that way.
[ld48] Build script now works.

Usage: ./build path/to/src/dir output_file.html

Requires twine to be checked out locally.
Substitutes includes if they're written in the following format:

    !!!include <path/to/include/relative/to/.tw/file>

 Then runs twee and redirects output to the specified file.