Adding UML sequence diagrams and embedded images into the generated HTML

[tysenmoore-xse]

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I've added the ability to embed PlantUML sequence diagram syntax in the source code comments and these will be converted to images and embedded into the generated documentation.

I've also made it possible to embed images or add links to images to your documents.

Closes #24

[prabhic]

Nice features love to have this feature in JSdoc also

[alerque]

This is interesting and potentially very useful, but it's going to need some cleanup before I can really consider it for merging. First we need to make sure it's completely optional and doesn't add dependencies. Second there seem to be some unrelated changes mixed in here in a few spots and perhaps some related but unnecessary bits too. Third, this needs rebasing.

I know its been a long time. Is this still relevant to you? Are you interested in working on it? Or is anybody else out there using this? It would be nice to have some real world use-cases to look at while testing it.

[tysenmoore-xse]

I'm afraid I have not touched this project is a very long time--nor Lua for that matter. This PlantUML feature was very useful to add readable text sequence diagrams to functions within my Lua code. When LDoc generated the documentation they were expanded into very nice PNG diagrams. I used my fork extensively in my previous work related projects. I think this would be a great feature to add to LDoc but I'm afraid I have been away from Lua too long and really don't have a use. It is a shame it has been so long.

For an example of a sequence diagrams in the Lua comments have a look at: https://github.com/tysenmoore-xse/LDoc/blob/master/tests/uml.lua. This is the test file I used at the time.

For some PlantUML sequence diagram examples you can go here: https://plantuml.com/sequence-diagram