Introduce {% startmermaid %} block template tag

[maiksprenger]

Motivation:

These changes implement the block tag suggested in #18

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[ArtyomVancyan]

Thank you, @maiksprenger, very much! You've made an incredible contribution, suggesting and implementing a new feature for more convenient use for the Jinja blocks. I have fixed the Django versions in the Tox environment, as well as a few code sanitizations. You can upgrade the package and use the feature you implemented :)

[maiksprenger]

@ArtyomVancyan Thank you for the kind words! This wouldn't have been possible without you having time to respond and review! And thank you for going the extra mile, and cutting a new release - that makes our lives much easier. Much appreciated!