Add take-damage method to player structure.
Quote jumps if he hits Yatty.
Enter a period of invincibility after a collision w/ Yatty.
Add invincibility timeout.

If Quote has recently taken damage he is marked as invincible.
During this time the `update()` loop will accumulate a timer for
3sec. After 3sec the invincibility flag & timer will both be cleared.

While the invincibility flag is set: Quote cannot collide w/ Yatty.
(Remove some whitespace errors)
Add flashing effect while player is invicible.

If (self.invincibility_timer / 50ms) is evenly divisible by 2 we skip the draw()
call for that frame.
Merge branch 'feature/react-bat-damage'