add plantuml activity diagram (new syntax) to templates

[mlthlschr]

Hi @npgrosser, thanks for the great plugin.
I was missing a template for the plantuml activity diagram, thus thinking it was not supported. It actually is supported by kroki :-) Would be nice to have it in the official plugin.

[npgrosser]

Thanks for the PR. The new template is now included in the latest release (0.0.11).

Note:
There are still some supported diagram types for which there is no template yet.
But I plan to add the option for users to create or modify templates themselves.