Custom field support

The biggest change is adding support for custom fields in the inventory. Rewrite the transformation function to be promise based because of async fetching the correct custom field type from the API. When the same type of custom field is being used multiple times it is only fetched once using memorisation.

Secondly the stock fields now have the same name as the API, "quantity" > "quantityOnStock" because header fields are no longer ignored but used to map the correct values. - Selwyn :)
Refactor and update mappings and CSV data handling.

The header row is not static anymore and thus includes a major breaking change, in both the naming of the fields as in the removed static header option.