Add support for rendering tW BTTWriter projects

[rbnswartz]

Currently only Bible is supported for BTTWriter projects. Would be nice to have this since it should be much easier now than it was in the original pipeline.
Examples:
http://read.bibletranslationtools.org/u/Staging/my_bible_tw/

http://read.bibletranslationtools.org/u/Staging/zh_bible_tw_l2/

http://read.bibletranslationtools.org/u/Staging/lo_bible_tw/