Make it possible to ignore annotations in certain files/directories #562

Maybe we need a configuration option to force certain modules to not be type checked (this would also allow arbitrary expressions in function annotations). This might be useful for legacy code that uses annotations -- and also for legacy code in general. The configuration option could be command line argument or a setting in a mypy configuration file, for example. By default, we could enable this for the Python std lib.

This was mentioned on python-ideas when discussing PEP 484.

An older idea is to not type check modules that don't import typing, but maybe that is too restrictive -- again, this was mentioned in the PEP 484 discussion.