External tools support

[kiith-sa]

Sometimes, we might want to include e.g. graps, math expressions, etc. in documentation. It would be best to be able to specify these in their languages (a text representation of UML, dot, metapost/asymptote, Processing) directly in DDoc.

We don't want to directly support such languages in harbored-mod to avoid non-D dependencies and just plain bloat (see Doxygen with its army of optional dependencies).

But we could add a syntax to specify sections to be processed by external tools, let user describe what these tools do (how they are used, do they generate HTML? image? file?). Then users with specific needs could specify the tools they need without making harbored-mod an unmaintainable mess.

E.g. something like:

_external-plantuml:   //Repurposing DDoc section syntax to avoid adding more syntax

plantuml source here

the _external prefix would be generic and could be followed by any name, and in hmod.cfg the user would have to explain what e.g. plantuml actually means and how to handle it (this will take a lot of work to get right; because e.g. some tools might generate an image that would be inserted, some might generate HTML that would be inlined directly, some might generate files that would be linked (e.g. PDF), etc.

Also, a completely separate "external tool description file" might be better than using hmod.cfg directly.