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

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