Task (anssel): Make use of extra supervision

[pasky]

In the yoda datasets of anssel task, we have extra supervision in the form of binary markers for tokens that actually denote the answer. We should try to make use of this supervision during training by just passing that as another set of NLP-style token flags. It should be pretty easy (except that it'll of course break transfer learning; but there's a way to keep transfer learning working if we make some changes that include fixing N to original embedding size).