Integrate third party BBCode writers in pandoc upstream.

[wiki-me]

This is the code in question

I also opened an issue with the third party repository mentioning why I think this is good for pandoc (besides making the writers more discoverable).

Bbcode is very useful, if I want to write a long forum post I can write it in markdown or ODT (that have good editors), and then convert it to bbcode and then post it.

[jgm]

We don't include lua writers in the main pandoc source code.
A Haskell version could be considered.