added get_features_by_ids

Hi Neurosynth,

I'd like to add a way to extract features, given a set of id's. An example use case: I have a Nifti-file mask, then get_ids_by_mask, and then get_features_by_ids.

This is helpful when I have a brain region and I would like to see the functions associated with it.

I would have tried to go directly from mask to features, but separating the features_table from the image_table seemed important since the length of the id-list is inconsistent between them.

Thanks,