Add BitFlags trait implemented by every bit flags

It would allow to use any bitflags as generic for example:

```rust
struct SomeStruct<F: BitFlags>{
....
}

impl<F: BitFlags> SomeStruct<F>{
    fn set_flags(&mut self, flags: F){
        ...
    }
}
```

it would also allow to document the crate without need for the example_generated module

I can send a PR implementing this if there's interest 