Add GetOrCreate<ListName>Map helper for lists.

[wenovus]

This helps avoid nil checks when there is a need to operate on the map field directly.

This is included along the other GetOrCreate helper func.

[coveralls]

coverage: 88.803% (-0.008%) from 88.811%
when pulling 2f02e9f on get-or-create-map
into 9cb751b on master.

[ejbrever]

Thanks!