Add support for structurizr includes.

[willson556]

Changes:

• Adds a map of diagram types to preprocessors.
• Adds a structurizr include preprocessor.

Resolves #11.

[alexander-bauer]

I've tried out using this branch tip, and it's working nicely for me. I'd love to see this merged to main. The code strikes me as in good shape, with these addendums (which might be outside of the scope of this PR):

1. It might be handy to expose the plugin configuration to preprocessors, if it's not already available through the global environment. I could see this coming up with cases like user macros, or preprocessor logic that uses external tools.
2. If there get to be more Preprocessors, then it might be handy for the preprocessor dict to contain a sequence of Preprocessors to be applied, rather than just one.