Increase snake's speed as its length increases

[carissaallen]

Overview

The snake's length increases with each apple it eats. So, as the player successfully eats more apples, it'd be neat if the game got a little more difficult by increasing the snake's baseline speed.

Acceptance Criteria

• The snake's speed increases at reasonable increments as it's length increases
• There is an upper bound on speed so the snake is never uncontrollably fast
• There is a lower bound on speed so the snake is never frustratingly slow
• Unit tests are written to cover the new code/methods added

Suggestion

Try looking at the window.timeout() functionality in curses. This may not be relevant to the final solution, but this method is what currently controls the delay in milliseconds that establishes the snake's speed.