Proxy definitions

[neithere]

Let's say we want to add a command to the parser with slightly different configuration:

class Foo:
    def meth(self):
        pass

foo = Foo()

command = argh.named('another-name')(foo.meth)    # WRONG!

dispatch_commands([command])

This is wrong because:

1. the code won't work because accessing the method's function is not the same as accessing the method (extra checks are needed);
2. the decorator (re)sets an object's attribute. If we only need to set a value right here, we should not touch the object itself; a proxy/wrapper should be applied instead.

For example (syntax to be improved):

@named('one')
def foo():
    pass

bar = proxy_named(foo, 'one')

assert foo.argh_name == 'one'
assert bar.argh_name == 'two'

The named decorator may be used for this purpose, too; it should be less transparent in that case.