Set jinja options from a config file in a template

As mentioned in my comment in #135, I've worked on a patch that allows jinja environment options to be specified in a config file in a cookiecutter template.

I put these options in a separate config file, so that older versions of cookiecutter don't ask for this as an option. I think this is going to break a lot of old versions of cookiecutter anyway if the cookiecutter templates start using these options :( I guess this requires some thought...