Advanced Indexing

Hi All,

Per my discussion with @neubig earlier today, it would be really great if we can have support for [advanced indexing](https://docs.scipy.org/doc/numpy-1.8.1/reference/arrays.indexing.html#advanced-indexing) feature in DyNet.

PyTorch also started [this issue](https://github.com/pytorch/pytorch/issues/1080) a while ago and seems to be officially supporting it soon in v0.2.0. Although this might look much too fancy for someone, I do know some people (like me) are using this prevalently for numpy tensors and back in the era of theano. Supporting such features would create a more seamless experience when these folks migrate to DyNet.

I hope someone who is familiar with the codebase can give some instructions towards how this could be integrated so people can work to add support for this feature.

Thanks!