Re-export and document BitFlags usage.

[Levitanus]

I found in tests.rs, that I cannot simply use BitFlags struct. I think, that I should include them as dependency in Cargo.toml, than try to construct in the test function.

I think — it is too complicated and not very clear for the end-user. Maybe consider re-export of BitFlags and writing a couple of strings in docs?

[helgoboss]

Yes, I think that's a good idea. At the moment one needs to include it manually making sure it's the same version.