add cli support for folders and output to a file/folder

[ZeeD]

Hi.
I've just found your tool and seems magic :) very cool

as I am working with a "big" schema definition (~100 .proto files, distributed in a 3/4 level nested tree) I was wondering if you could enhance pb2py to support the input from a folder (and implicitly convert all the *_pb2.py files inside) and output to a target folder (and not on stdout)

at the moment I am putting in place a shell script based on find, mkdir -p and redirection, but it's very fragile and I was wondering if you could help on the tool side