* Lua: allow access to pandoc state
Lua filters and custom writers now have read-only access to most fields
of pandoc's internal state via the global variable `PANDOC_STATE`.
* Lua: allow iterating through fields of PANDOC_STATE
* Lua filters doc: describe CommonState
* Lua filters doc: mention global variable PANDOC_STATE
* Lua: add access to logs
Log messages can currently only be printed, but not decomposed.
