[Resolve Sceptre#746] add before_launch and after_launch hooks

docs/_source/docs/hooks.rst

@@ -18,6 +18,8 @@ Hook points
 
 ``before_delete`` or ``after_delete`` - run hook before or after Stack deletion.
 
+``before_launch`` or ``after_launch`` - run hook before or after Stack launch.
+
 Syntax:
 
 Hooks are specified in a Stack’s config file, using the following syntax:
@@ -199,4 +201,4 @@ This hook can be used in a Stack config file with the following syntax:
 .. _Custom Hooks: #custom-hooks
 .. _subprocess documentation: https://docs.python.org/3/library/subprocess.html
 .. _documentation: http://docs.aws.amazon.com/autoscaling/latest/userguide/as-suspend-resume-processes.html
-.. _this is great place to start: https://docs.python.org/3/distributing/
+.. _this is great place to start: https://docs.python.org/3/distributing/

sceptre/plan/actions.py

@@ -158,6 +158,7 @@ def cancel_stack_update(self):
         )
         return self._wait_for_completion()
 
+    @add_stack_hooks
     def launch(self):
         """
         Launches the Stack.