Stop injecting MypyStatusItem in pytest_collection_modifyitems to fix --looponfail

[dmtucker]

Resolve #85

This behaves much better.

• Using --looponfail should no longer crash or run duplicates.
• The status check can now be run by node ID on any file (though, it may not be a really useful thing to do).
• Injecting just MypyFileItems in a pytest_collection_modifyitems hook no longer implies a status check.
  If you want that, you need to inject a MypyStatusItem too.
• Likewise, the MypyStatusItem is now present in items in pytest_collection_modifyitems.
  Remove it to ignore mypy errors in uncollected modules.