add option to ignore out of line modules

[matklad]

Fixes #400.

I'm not sure how should I add a test for the option :)

[nrc]

To test an option you add a comment like this to your test.

[nrc]

I guess the tricky thing with tests is that if there is a sub-module that we expect not to be formatted, then it will appear in the target dir as unformatted, however, the test infra when finding the file as a top-level file will want to reformat it, so there will be an error.

I think the solution is to add a don't test directive to the testing infra which can be added to a file and will skip testing for that file as a top-level file, but not as a sub-module. Although tbh, I don't think our testing infra is module aware in any case, so maybe we can't test this at all.

[nrc]

OK, just going to merge, who needs tests any way? If you think of a way to test this or want to modify the testing infra, that would be awesome, but I don't want to block this on that.

[nrc]

Thanks for the PR!