lua: Added simple version of lua destination.

Usage:

  destination d_lua {
    lua(
      script("test.lua")
      init-func("test_init")
      queue-func("test_queue")
      template("$MESSAGE")
    );
  };

Parameters:
 * script: the name of the Lua file which contains the functions for the
   destination.
 * init-func: the name of the Lua function which is called during
   initialization of the destination.
 * queue-func: each message reaching this destination is formatted using
   the template parameter, and passed to this function as a Lua string.
 * template: can be omitted, it's default value is "$MESSAGE"

Each lua destination has it's own state, so init and queue functions can
have side effects, and global variables are kept between init and
queue calls.

Example lua file:

--BEGIN
function test_init()
  print "Init called!"
  counter = 0
end

function test_queue
  counter = counter + 1
  print(msg  .. " " .. tostring(counter))
end
--END
README.md: Add a pointer and some notes about the Lua destination

Signed-off-by: Gergely Nagy <algernon@balabit.hu>