perform mget during suggest. requires suggester output to be a gid

READY, BUT DO NOT MERGE YET!

This commit should be tagged as a v2.x release as it breaks backward compatibility.

This branch changes the `/suggest` behaviour to work as discussed at the bottom of this gist:
https://gist.github.com/missinglink/0b9c9d01bfdf6e95e76a

It breaks backwards compatibility as we previously returned `payload` and a human readable `output` from `_suggest` but since https://github.com/pelias/suggester-pipeline/pull/12 we don't return the same info.