Setup extras so Jinja2 isn't a hard dependency

[sontek]

Since Jinja2 is an optional dependency it would be nice if it wasn't forced to be installed. I want to ship mr.bob with a library I'm creating but I use Mako instead of Jinja2

[glenfant]

This will require changes in setup.py (and perhaps others) to existing mr.bob based templates (bobtemplates.plone, etc.) that provide the default Jinja2 templates 👎