Lua: add function pandoc.utils.documentation

[tarleb]

The function allows to retrieve the documentation of an object. It can be used during interactive development and to generate API documentation.

[tarleb]

This works, but is incomplete, as only ANSI output is supported right now. I have to shelf this for a couple of days, and the PR is intended to allow some preliminary testing and gather feedback.

[tarleb]

I'll consider this PR ready if it allows to replace most of tools/update-lua-module-docs.lua with calls to pandoc.utils.documentation.

[tarleb]

Most of tools/update-lua-module-docs.lua is still required, but just for the main pandoc module, and automatic docs generation is disabled for pandoc.zip and pandoc.layout, all of which are a bit special.
Documentation for all other modules is now generated through the new function.

Here's a screenshot of running pandoc.utils.documentation(pandoc.read):

I consider this PR ready.

[jgm]

Great!