Add support for blockdiag, nwdiag, and actdiag

[moigagoo]

This is absolutely trivial since seqdiag is already there.

http://blockdiag.com/en/

[moigagoo]

I refactored the diagram processing routine, so adding new diagram backends should be even more trivial now (ref).

[moigagoo]

Whoever will implement this, please refer to this tutorial: https://github.com/foliant-docs/foliant/wiki/Adding-a-Diagram-Backend

[Vanderhoof]

Done: https://foliant-docs.github.io/docs/preprocessors/blockdiag/