Add csv and text export handler

Plugins may generate and display data in their UI, but have needed an
option to be able to export that data.  This handler will echo back an
JSON array of arrays as a csv file, or for more complicated/custom
structures will also simply initiate a download of the posted text.
Merge pull request #282 from CoastalResilienceNetwork/topic/csv-export

Add simple csv and text export handler