Add popular online diagramming tools: diagrams.net and lucidchart.com

[WBSTRAT]

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[waldyrious]

Lucidchart has been added as part of #52.

As for diagrams.net, I am not sure how closely it meets the criteria of an infinite canvas. Yes, the space is unlimited, but they definitely work with a notion of fixed page sizes, even though new "pages" do get automatically enlarged if you move beyond the current borders:

(Note how they have a separate notion of "page" that's orthogonal to this "virtual paper size" feature.)

In contrast, Lucidchart is much more fluid in how they handle the page size:

I personally don't think it's a big deal, especially since the page enlargement in diagrams.net is automatic, but I guess it's up to @arun-is to decide how strictly the criterion of expansiveness should apply for a tool to be considered a proper implementation of the infinite canvas concept.