Ensure that unmapped CSV fields are imported into the 'extras' dict.

Currently, Go's CSV importer has a bug where if you don't select a mapping for a CSV field, it doesn't get put into the 'extras' dict for the imported contacts.

The cleanest solution would to add a mapping type "Other" or "Extra" so that the user must explicitly decide whether to import a certain field that we can't auto-detect a mapping for. 

Here's the actual cause of the bug, where we basically end up nilling out the field name for columns which the user doesn't map.
https://github.com/praekelt/vumi-go/blob/develop/go/contacts/templates/contacts/includes/tools.html#L132