add Line constructor

Line.py

@@ -1,3 +1,7 @@
 
 class Line(Shape):
-    pass
+    def __init__(self, x0, y0, x1, y1):
+        self.x0 = x0
+        self.y0 = y0
+        self.x1 = x1
+        self.y1 = y1