Implementing sass_free_folder_context() function.
Implementing sass_free_folder_context() function.

Although sass_interface.h header contains the prototype of
void sass_free_folder_context(struct sass_folder_context *)
function, sass_interface.cpp hadn't implemented it.
This had caused link errors.

dahlia/libsass-python,admire93/libsass-python,admire93/libsass-python,dahlia/libsass-python