Implement Export Name column for data

[mradamcox]

To address #158, we have added a new field, Export Name to the Columns sheet in the data dictionary and variables Google Document. This field allows us to link the Column field (which I would like to rename to "Source Column") to a shapefile-compliant 10-character field name that will be used in the exported dataset.

To implement this field, we need

• [x] The public data build must transform Source Column names to Export Names before writing the shapefile and geojson output files.
• [x] The Data Dictionary list must display the Export Name for each column.

[mradamcox]

I just did this as part of something else.