Lua module: add readers submodule

[tarleb]

Plain text readers are exposed to lua scripts via the pandoc.reader
submodule, which is further subdivided by format. Converting e.g. a
markdown string into a pandoc document is possible from within lua:

doc = pandoc.reader.markdown.read_doc("Hello, World!")

A read_block convenience function is provided for all formats,
although it will still parse the whole string but return only the first
block as the result.

Custom reader options are not supported yet, default options are used
for all parsing operations.

[jgm]

Can you say a bit about the intended use of this? An example that motivated this addition?

[tarleb]

I had two use-cases in mind: reading and handling additional metadata (#1960, #3115) and creation of custom include mechanisms for formats that normally don't support it.

Long term, it could become possible to write conversion scripts, allowing for fully portable programs without much trouble. Something like nmap's NSE would be nice.