Add headers to the sheet

I don't think it is something worth implementing. Two factors I considered:
- Spreadsheet is created once and module is configured once. It isn't that hard for person managing Drupal and Google API to add bunch of columns.
- I found myself adding removing fields/columns and renaming the headers multiple times.

That being said, if someone drops a patch with decent implementation, why not.

Closing this until someone comes with a good solution.