Use horizontal layout in export modal to save space for column controls
Add tabs to separate the columns from the other export options
Override Cilantro styles to increase size of the export modal

I am not sure why the !important is needed. I can't seem to get this to
override the Cilantro style any other way regardless of import order.
Add support for choosing columns when exporting
Merge pull request #34 from cbmi/issue-29

Allow user to select columns when exporting