Draft: Extend syntax to allow ignore annotation on tests

[svenfoo]

This is a draft, the implementation is tested, but can certainly be improved. But I would like to ask for comments about the syntax first and whether this "feature" would have a chance to be accepted at all.

Bear with me, this is the first time I ever touched a Rust macro and also the first time I got in contact with syntax trees.

Supposedly fixes #7.