Support exclusion of specific imports

[tjstum]

Sometimes we know that a particular import always has side effects and should never be removed. When running autoflake with --remove-all-unused-imports, there's not a way to say "everything except module X, Y and Z."

We can sprinkle # noqa around at each such sight, but I'm wondering if there would be appetite to support a flag like --never-remove or such (that's the inverse of --imports).

If there is some way to do this that I am unaware, please let me know and accept my apologies! If this is intentionally unsupported, that would also be helpful feedback. If this is of interest but just needs a contributor, I could take a look.

Thanks!

[fsouza]

I think this is reasonable. One question I have is whether you're using flake8 though? Just because we may be able to ignore it in autoflake, but if you're using flake8 it may still require the issue. Not sure if you care.

[tjstum]

In our particular case, we are not otherwise using flake8.