Adding external itemset support

This pull request adds support for the external itemsets described here: http://opendatakit.org/help/form-design/external-itemsets/

To use external itemsets the user needs to add an "external_choices" sheet to the workbook with the options they will reference, then add a "select one external [list_name]" type prompt to the survey sheet with a choice_filter column. An example workbook is included in at: tests/example_xls/select_one_external.xlsx
The external_choices sheet needs to be converted to an itemsets.csv file that is included in the xform's media folder. The CLI will automatically output it as itemsets.csv in the same directory as the xfrom.

While adding this feature I also cleaned up some duplicate definitions of aliases and used the argparse library to simplify the CLI code.