lua-font-lock-keywords: rewrite poorly written regexp (issue #47)

Said regexp caused O(4^N) match attempts on lines containing N
underscores, because underscore had 'word' syntax class.
Revisit block-close unindentation logic (issue #26)

- completely rewritten left-shifter expression regexp
- reduced extra parsing work in cases when block-start token is already
  at (current-indentation).
Put Lua reference resources into TODO file