Mkdocs-mermaid2-plugin

* Added: a test framework with MkDocs-Test and pytest
* Changed: migrated from `setup.py` to `pyproject.toml`

* Changed: If the `javascript` parameter starts with http(s) and no Internet
access is available, a WARNING is now issued
(mkdocs no longer fails with an exception).

* Fixed: Bug with local javascript library

* Added: Parameter `javascript` in config file for optionally specifying the
URL or path of the Mermaid javascript library.

* Changed: Parameter `extra_javascript` in config file is DEPRECATED,
for optionally specifying the URL or path of the Mermaid javascript library

* Changed: Updated documentation.

* Fixed: Arguments of config file not taken into consideration,
for mermaid.js version > 10 (82)

* Added: A new [doc website is available on Read The Docs](https://mkdocs-mermaid2.readthedocs.io/en/latest/).