* Adds message references to message string interpolation, in the form of %{BKY_STRING}.
* Re-adding CONTROLS_IFELSE block using the new syntax, referencing to CONTROL_IF equivalents.
Adding controls_ifelse, an if/else block that is loaded from JSON and does not use mutators. This gives "else" capability to Android & iOS implementations, which don't support JavaScript mutators.
Added this block to the playground simple toolbox and all generators.
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.
