Enable template includes ({% include 'foo.txt' %}.

The original idea of the `render` function was to have an interface to envtpl that you could just pass a string and some variables and get the same result as you would on the command line. I'm not sure anyone used `render` in that way, other than me in some old project. I can't think of a nicer way of allowing includes right now, so let's just make `render` => `_render` so it at least isn't exposed in the API.