Support three-argument subtraction (gh-training#8)

Users have reported that 3-arg subtraction doesn't work
We forgot to implement it!

src/calc.py

@@ -9,12 +9,12 @@ def add(a, b):
     """
     return a + b
 
-def sub(a, b):
+def sub(a, b, c = 0):
     """
     Subtract some numbers
 
     ```py
     sub(8, 3) # 5
     ```
     """
-    return a - b
+    return a - b - c

src/calc_test.py

@@ -6,10 +6,13 @@ class TestStringMethods(unittest.TestCase):
     def test_add_2arg(self):
         # Make sure 3 + 4 = 7
         self.assertEqual(add(3, 4), 7, 'adding three and four')
-   
+
     def test_sub_2arg(self):
         # Make sure 4 - 3 = 1
         self.assertEqual(sub(4, 3), 1, 'subtracting three from four')
 
+    def test_sub_3argument_variant(self):
+        self.assertEqual(sub(4, 3, 1), 0, 'subtracting one and three from four')
+
 if __name__ == '__main__':
     unittest.main()