A controllable snake!

A controllable snake!

Added snake in README!

Day 20/README.md

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+# Day 20 - Build the Snake Game Part 1: Animation & Coordinates
+
+- Made a controllable snake, and will complete the full snake game on Day 21.
+- Abstracted snake logic into a Snake class.
+
+## Snake
+
+![Day_20_Snake](https://user-images.githubusercontent.com/30333942/129074881-fff5a265-a4a7-445c-93bc-fc6553c50166.gif)

Day 20/main.py

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+from turtle import Screen
+from snake import Snake
+import time
+
+screen = Screen()
+screen.setup(width=600, height=600)
+screen.bgcolor("black")
+screen.title("A Snake Game!")
+screen.tracer(0)
+
+snake = Snake()
+
+screen.listen()
+screen.onkey(snake.up, "Up")
+screen.onkey(snake.down, "Down")
+screen.onkey(snake.left, "Left")
+screen.onkey(snake.right, "Right")
+
+game_is_on = True
+while game_is_on:
+    screen.update()
+    time.sleep(0.1)
+    
+    snake.move()
+
+screen.exitonclick()

Day 20/snake.py

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+from turtle import Turtle
+STARTING_POSITIONS = [(0, 0), (-20, 0), (-40, 0)]
+MOVE_DISTANCE = 20
+UP = 90
+DOWN = 270
+LEFT = 180
+RIGHT = 0
+
+class Snake:
+
+    def __init__(self):
+        self.segments = []
+        self.create_snake()
+        self.head = self.segments[0]
+
+    def create_snake(self):
+        for position in STARTING_POSITIONS:
+            segment = Turtle(shape="square")
+            segment.color("white")
+            segment.penup()
+            segment.goto(position)
+            self.segments.append(segment)
+
+    def move(self):
+        for segment_index in range(len(self.segments) - 1, 0, -1):
+            new_x = self.segments[segment_index - 1].xcor()
+            new_y = self.segments[segment_index - 1].ycor()
+            self.segments[segment_index].goto(new_x, new_y)
+        self.head.forward(MOVE_DISTANCE)
+    
+    def up(self):
+        if self.head.heading() != DOWN:
+            self.head.setheading(90)
+
+    def down(self):
+        if self.head.heading() != UP:
+            self.head.setheading(270)
+
+    def left(self):
+        if self.head.heading() != RIGHT:
+            self.head.setheading(180)
+
+    def right(self):
+        if self.head.heading() != LEFT:
+            self.head.setheading(0)