Make directory processing more permissive

[shamilsan]

Sometimes we are to open CFB files with corrupted directory structure.

In this PR we skip weird dir entries (in permissive mode) instead of returning an error.

Mostly inspired by compoundfiles.entities

[shamilsan]

@mdsteele Could you please take a look?

[mdsteele]

Thanks. Could you add some unit tests for this?

[shamilsan]

Thanks. Could you add some unit tests for this?

Ok, I'll try my best a little bit later.