The new "stripped" argument allows for more expressive multi-line values

passed to {% definevar %} without worrying about the leading or

trailing whitespace that would occur. The argument will strip away the

whitespace before storing the value in the context.
This required a new resolve_vars option to the @blocktag decorator,

which controls whether the arguments passed to the template tag function

will be automatically resolved as variables. By default, they continue

to be resolved, but this behavior can now be turned off.

Unit tests pass.