Filtering functions take element components as arguments instead of the
whole block elements. This resembles the way elements are handled in
custom writers.
* Add `--lua-filter` option. This works like `--filter` but takes pathnames of special lua filters and uses the lua interpreter baked into pandoc, so that no external interpreter is needed. Note that lua filters are all applied after regular filters, regardless of their position on the command line.
* Add Text.Pandoc.Lua, exporting `runLuaFilter`. Add `pandoc.lua` to data files.
* Add private module Text.Pandoc.Lua.PandocModule to supply the default lua module.
* Add Tests.Lua to tests.
* Add data/pandoc.lua, the lua module pandoc imports when processing its lua filters.
* Document in MANUAL.txt.
