mongodb river for elasticsearch
There is a new MongoDB river on github:
https://github.com/richardwilly98/elasticsearch-river-mongodb
according to the code you can specify several things but there is no separate doc (expect one mailing list discussion):
This isn't really the answer you're looking for. I looked at building this mongo river but I found some discussion on it having some memory leaks and I didn't want to fiddle with Java code. I wrote my own mongo->ES importer using the bulk API.
It's a work in progress, so feel free to contribute! :)