Support for splitting multi-select columns in custom exports

depends: https://github.com/dimagi/couchexport/pull/108

![multiselect_export](https://cloud.githubusercontent.com/assets/249606/5317203/51c7d912-7c9d-11e4-89e5-ed4a458e94e2.png)

Not crazy about this approach but other options seemed orders of magnitude more complex