lua: Added simple version of lua destination.

Usage:

destination d_lua {
  lua( script("test.lua") init("test_init") queue("test_queue") template("$MESSAGE") );
};

Parameters:
 * script: the name of the Lua file which contains the functions for the destination.
 * init: the name of the Lua function which is called during
initialization of the destination.
 * queue: 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 keeped 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