Added before_each and after_each

[Ledmington]

Hello, i used this library two times for my projects at university and it was absolutely a game changer. Great work!
I like this library and i really want to improve it. This pull request is actually the first of a series that i'll be doing in order to add some useful features. This one adds the before_each and after_each functions, as they are called in JUnit for reference, in substitutions of setup and teardown. These two have become, respectively:

• the function to be called before the very first test of the suite
• the function to be called after the very last test of the suite

I am willing to help as much as possible to improve MinUnit and my pull request, so please let me know if you want me to change anything.

I can also provide a file with a simple example usage, if you want.