Add --lowercase option to map column names to lowercase struct names

[ehalpern]

By default, each column name maps to a struct field preserving the original case. If --lowercase is specified the resulting struct fields will always be lowercase.

[ehalpern]

This feature is motivated by the work to import taxi data (http://www.nyc.gov/html/tlc/html/about/trip_record_data.shtml) into noms. The case conventions for csv columns are inconsistent across files. The best way to avoid complication and false duplicate fields is to normalize to lowercase.

@cmasone-attic, @kalman

[cmasone-attic]

lgtm