Support a generic numbered directive with user defined name and other properties

[shailesh1729]

Is there a plan to support a generic numbered directive where the actual name and color of the directive can be user controlled? In LateX one can create new theorem environments with whatever name one chooses. I keep thinking of a few more directive types like "fact", "hypothesis", "exercise" "challenge", etc.. Some directives are too specific to a particular domain [e.g. "constraint qualification" which is specific for Lagrangian duality].

Originally posted by @shailesh1729 in #80 (comment)

In some sense, this will reduce the documentation work as users will be free to create their own directives.