Support ignored tests

[korrat]

This PR adds a new argument ignore to the attribute macro, which expands into an #[ignore] attribute on the generated test function. A minimal test for this functionality is included, but no tests for the interaction with other arguments or features yet.

Additionally, this PR includes attributes specified on the input function in the generated test function. If you'd like, I can break this functionality out into its own PR.

With the second change, it should also be possible to remove the first change and deprecate the should_panic argument in favor of specifying #[ignore] or #[should_panic] directly.

Fixes #12

[korrat]

ping @mgattozzi