Provide `main()` function in App subclasses

[kalekundert]

An app should have a main function. This function should be defined something like so:

class MyApp(appcli.App):
    @classmethod
    def main(cls):
        app = cls.from_params()
        app.do_something()

This problem is what to call the do_something() method. It's basically the main function that the user themselves writes, and so it also wants to be called main(), but I think it makes the most sense to reserve the name main() for the static method that actually can be used as an entry point. So, my options are:

• Hard-code a name like do_main() or _main(). The do_ prefix is my normal idiom for this kind of thing, but it's unattractive.
• Provide a @main decorator. I'd have to do some error checking (e.g. exactly one main function), but it stays in the background.
  • Because I already have a metaclass, I could inject the @main decorator directly into the namespace where class methods are evaluated. That would let it save a direct reference to the method, so I wouldn't need to search through every defined method for a hit.
  • I'd have to think about how to implement the injection idea in a way that suports inheritance. Maybe I would ultimately have the decorator be sugar for some more explicit method of specifying the main function (e.g. hardcoded name).
• I could name the static method entry_point(), then use the name main() for the regular member function.

[kalekundert]

Fixed in f7e865a