Support for splitting multi-select columns in custom exports

depends: https://github.com/dimagi/couchexport/pull/108

This is behind a toggle so could be QA'd on prod

![image](https://cloud.githubusercontent.com/assets/249606/5355067/18d3e878-7f98-11e4-9235-a733730ab48d.png)


This is likely overkill but hey