Re-commit partial completion of #42: Refactor docstrings according to PEP 257 using "pep257":
* defaults.py: template_session
* filters.py: pop_self
* filters.py: fill_empty
* helpers.py: *_app
* helpers.py: console_command
* helpers.py: get_kwargs
* helpers.py: custom_import
* helpers.py: to_func
* helpers.py: to_json