feature: Display file diff on black-check (aws#3554)

Author: claytonparnell

tox.ini

@@ -138,7 +138,7 @@ setenv =
     LC_ALL=C.UTF-8
     LANG=C.UTF-8
 commands =
-    black -l 100 --check ./
+    black -l 100 --diff --color --check ./
 
 [testenv:clean]
 skip_install = true