Positional argument decorator

[henryiii]

​Adding positional arguments decorator to plumbum.cli. This allows positional or named validators to be added via a decorator on the main function. Default arguments do not go through the validator. For example:

class MyApp(cli.Application):
    @cli.positional(int, float, cli.ExistingFile)
    def main(self, myint, myfloat, *inputs):
        assert isinstance(myint,int)
        ...

This also supports Python 3 notation:

class MyApp(cli.Application):
    def main(self, myint : int, myfloat : float, *inputs : cli.ExistingFile):
        assert isinstance(x,int)
        ...

Also fixed a bug with missing __slots__ in ABC, causing ABC subclasses to be __dict__ classes.

Besides convenience and clarity, especially for variable args, this is going to be important for 1.7.0 if we can get argument completion (since the classic method of manually validating each argument inside main is not introspectable). I have not changed the help output, but that is another future possibility.

Closes #200 (my own request)