Allow multiple bitflags in one macro invocation

[konsumlamm]

Add support for multiple bitflags in one macro invocation, for example:

bitflags! {
    pub struct Flags1: u32 {
        const A = 0b00000001;
    }

    struct Flags2: u32 {
        const B = 0b00000010;
    }
}

The changes to the macro are backwards compatible, so I changed the version to "1.3.0".
I also updated the documentation and replaced mentions of the flags keyword (which was used before 0.9) with struct.

[konsumlamm]

Note that the CI is only failing, because the expected error message for test_suite/tests/compile-fail/private_flags.rs is different on nightly. That is not caused by this commit. See #213.

[konsumlamm]

Any opinion on this?

[konsumlamm]

Closed in favor of #235.