added area of three points

I am going to use the area of triangles when determining if the point is
inside of another square to determine containment.  This was just an
important step

triangle.go

@@ -0,0 +1,7 @@
+package rect
+
+import "math"
+
+func SizeTriangle(p1, p2, p3 Point) float64 {
+	return math.Abs((p1.X*p2.Y + p2.X*p3.Y + p3.X*p1.Y - p1.Y*p2.X - p2.Y*p3.X - p3.Y*p1.X) / 2)
+}