googlesheets plugin feature: Ignore top n rows on import

Jes

Reading a google sheet with the plugin currently requires that header columns are in row 1. In the wild, a lot of users don't build sheets like that and the data begins some rows down the sheet. I suggest to add a feature of ignoring a number of top rows to correctly set the header row and table data.

Emiel

Since this request often pops up in our org, I created this pull request.