First draft of template engine chapter

I've added myself to the table of contributors, but I have a few questions about the information we're choosing to present.
We might not want to refer specifically to `coverage.py`'s use cases here, as I think this is the first time we're mentioning that project's name. You could mention early on that this work is inspired by `coverage.py`; alternately, a more general discussion of strengths and weaknesses of both approaches could include this example.

I think it might actually be good to explicitly ask the question "When might one approach be better than another? It depends on how you're using your templates" or something like this.