specifying field types for csv-schema command

Problem:
Sometimes the infer-schema command will infer the incorrect type (eg it might see a zip code like "12345" and call it a long, when other zip codes might look like "12345-1234", so it should be a string).

Solution:
Allow users to specify field types as args to the csv-schema command using an optional "--field-type field_name=field_type" parameter. If a field is found with the supplied name, its type is assigned based on the supplied value rather by being inferred.