Jinja2 Strict Mode

[plombardi89]

Jinja2 should error when it sees a variable in a template that it does not have defined. This would prevent empty interpolations from occurring.

[rhs]

Thanks for pointing this out, I didn't realize it existed. I've enabled warnings for now in 0.3.2 since this is an incompatible change, but I'll turn it into an error for 0.4