* fix(diagrams): Set a dummy filename to prevent raise error when both name and filename are not provided(#203)

* Update __init__.py

Update default name to "diagrams_image"

* Update test_diagram.py

Update unit test to reflect the file name change.

* Edges support

* Documentation and additional style attribute

* Better example and review changes

* Fix xlabel

* Fix docs

* Add Edge default properties

* Fix edge docs

Co-authored-by: Andrew Selivanov <a_selivanov@wargaming.net>

This allows you to have a consistent output filename, even if you decide
to tweak the diagram name.