Add on_exit() hook

[charlesej]

This update adds support for an on_exit() hook.

Overview of changes contained in this pull request:

• Support for users to define an on_exit() function which is called after the game loop is exited. This function can be defined with 0 parameters or 1 parameter (error_status).
• Support for users to define an on_enter() function which is called before the game loop is entered.
• Existing exit() function has been changed to use the common exiting path.

Notes:

• Felt using the name on_exit() seemed better than on_before_exit().
• Added on_enter() support for symmetry.
• Documentation still needs to be updated for this. I can add that to this pull request or open a new issue to track it.
• Could only find a few test cases for PGZeroGame (in test_event_dispatch.py). If you would like test cases for these changes, I can add them to this pull request or open a new issue to track them.

System details (used for testing):

• os: windows 10
• python: 3.5.0
• pygame: 1.9.4
• pgzero: master branch at ef963beaa22db89cb011695e96587bb268cbec2a
• numpy: 1.15.4

Resolves #51.

[penguintutor]

It looks like this issue has stalled. I've seen someone else asking for this feature on reddit.

I don't see much need for a status code for most users. Perhaps there is someone that it may be helpful to, but it think a simple on_exit() would be sufficient.

I'm happy to have a go at making the changes and creating a pull-request, if that will help.