MINOR: lua: Add tokenize function.

MINOR: lua: Add tokenize function.

For tokenizing a string, standard Lua recommends to use regexes.
The followinf example splits words:

   for i in string.gmatch(example, "%S+") do
      print(i)
   end

This is a little bit overkill for simply split words. This patch
adds a tokenize function which quick and do not use regexes.