SIBI-99: Added the ability to add and remove columns from the export.

Hi @joaomachiana,

Please review and test the code below and if everything looks good, you can merge this merge this pull request into the master branch. I've added the latest build of (the configurable-columns) branch of this module to the [SIBI Vagrant instance](https://github.com/jembi/sibi), so you can test by pulling my changes there and booting up the instance.

Cheers,
Pascal