How to treat code generation?

[Klafyvel]

Code generation is the most direct application of this package, hence it has been included almost since the beginning. However, if other code generation targets were to be added, we need to decide where to put them: here or in a separate repository.

The way I see things right now:

• The main code generation utilities can stay within Norg.jl
• Pandoc JSON could be in its own repository.
• HTML generation is quite valuable when working in notebooks, but to reduce dependencies, we could use the new weak references to make it available only if Hyperscript.jl is available.