feat: Add #[ignore] macro (#139)

Resolves #16.

After some discussion on #16
I went with the approach of accepting attributes following `#[test]`
(for now only `#[ignore]` is accepted), as this would allow us to accept
the standard syntax `#[ignore = "some reason"]` (and later
`#[should_panic = "some expected message"]`). These are not allowed in
custom attributes, but so long as we don't emit these (only parse them
internally) it's all good.

Author: epage