Additional visualisation parsers for MarkDown code blocks

Similarily to the infamous [StackEdit](https://github.com/benweet/stackedit) and its MathJax and graph integrations, HackMD already integrates similar visualisation languages, which can probably be extended by the following:

* https://github.com/bramp/js-sequence-diagrams
* https://github.com/adrai/flowchart.js
* http://jumly.tmtk.net/

If one would manage extended file upload capabilities, i.e. also for raw data files that could be served to an inline version of the declarative syntaxes of the following, one could also think about including:

* https://github.com/amitkaps/visdown
* https://github.com/philogb/jit
* https://github.com/almende/vis
* https://github.com/jacomyal/sigma.js
* https://github.com/cytoscape/cytoscape.js

This would bring HackMD even closer to the Jupyter Notebook ecosystem.

The issue names several JavaScript visualization libraries but no repository files, tests, or entry points. First identify the existing Markdown code-block integration and decide which visualization languages are in scope; done would mean the selected integrations render correctly in CodiMD, with the related upload capability clarified.

Scritto dal modello di indicizzazione a partire dal testo della issue.