0.21.0

0.20.1 -> 0.21.0

Additions:

• The enemies are now separated into distinct waves.
• There are now messages denoting the start and end of each wave.
• There is now a score displayed at the end of the game. 
  • Those who are curious how the score is formed, can see the detailed breakdown in the browser console.

Changes:

• Enemies killed by the special defenders will now grant half of the money you would otherwise get. This is to ensure that a situation where a wave breaking through leaves you unable to continue.
• The messaging system was re-written to be more maintainable.
• The endgame and victory/defeat states are determination logic was updated to account for waves.