Disallow f(x, y=1, z) and similar nonsense

Disallow f(x, y=1, z) and similar nonsense

Python rejects such function definitions because it doesn't make much
sense to have arguments with no default values after arguments with
default values.

Jinja did allow them, but handled them improperly (associating the
default value with the wrong argument).

Due to how broken the current behavior is, it makes more sense to reject
templates containing such defintions instead of trying to handle them
properly. Both cases are going to break existing code containing such
definitions, but besides the fact that possibly no such code exists it
is better to fail with a clear error than to silently change the values
of arguments.

This fixes #364