Add an option to not filter useless pass statements

[qyearsley-zz]

In fix_code, filter_useless_pass is always called.

But, it would be nice to be able to remove unused import statements without modifying pass statements, and to control removal of pass statements separately.

[myint]

I think this is related to #9.

[qyearsley-zz]

Ah, that's right! Yep, if that pull request can be modified to add pass statements where they're required when imports are removed, and also keep useless pass statements elsewhere, that would be what I had in mind with this issue.