Added parsing of ignore annotation for import statements.

This implements @JukkaL's suggestion of adding `# mypy: ignore` annotations in #500.

In this PR, only ignore annotations on import statements are allowed, but I think it would be easily extendable to other statements as well. Every statement `Node` could have an `annotations` field that could be used during semantic analysis or type checking.
Good to see that I'm not the only one who cares about theses things :smile: