Optionally don't enforce checker on `__init__.py`

[arclabs561]

__init__.py shouldn't be considered code, or at least it should be considered differently.

It doesn't make sense to enforce print_function, division on a file that only contains imports, a docstring, and perhaps __all__.

Perhaps a better option would be, don't enforce for an __init__.py file only having some subset of {imports, docstrings, comments, all = ...}?

Another option would be to have a separate --select or --ignore for __init__.py files.

[cleder]

require-code=True solves this for empty __init__ files

[labrys]

If you don't want a specific error on non-empty __init__.py just add it to your per-file-ignores.

[DylanYoung]

Should the require-code option also ignore imports?