Closes #470

This PR introduces a #[flag_name] attribute you can apply to flags to give them a different name. I've tried to avoid adding these kinds of attributes in the past, but there's no way to get around this one with bitflags, so we support it. It lets you write:

bitflags! {
    pub struct MyFlags: u8 {
        #[flag_name = "a"]
        const A = 1;
    }
}

There isn't any validation done on the flag_name given, so you could pick strange names like _, or a | b or a, etc. I need to add some more test coverage before this is ready.