Add flag support to deadline scheduling.

[FridayOrtiz]

Regarding the discussion in https://github.com/vityafx/thread-priority/pull/12, the crate currently does not support setting flags during deadline scheduling. I would like to add it.

@vityafx , I don't have permissions to assign issues in this repo but feel free to assign it to me.

[iddm]

Done! Thanks for the help!