Add a Lua code generator

This is based on the JS generator, with a lot of inspiration from Ellen
Spertus's blockly-lua: https://github.com/espertus/blockly-lua

All unit tests pass with Lua 5.3.2.

appengine/README.txt

@@ -30,6 +30,7 @@ blockly/
      |- blocks_compressed.js
      |- dart_compressed.js
      |- javascript_compressed.js
+     |- lua_compressed.js
      |- php_compressed.js
      `- python_compressed.js