support reading Argparse instances inside classes

[laf0rge]

So far, shphinxarg only has support for reading/importing Argparse instances from global variables/attributes within a module. However, there are use cases where Argparse is used inside classes of a module, not as a global variable. This is particularly the case for uses of 'argparse' in the context of CLI / REPL style user interfaces, such as for example those built using the 'cmd2' module.

This change introduces the ability to specify :func: wit '.' notation like foo.bar.baz