allow ... to be passed to `rmarkdown::pandoc_convert` in `gt_save_docx` (`gt_save`)

[katossky]

At the moment the structuration of gt_save forbids passing arguments to the inner rmarkdown::pandoc_convert through .... (The ... is unused in the intermediate function gt_save_docx.)

Yet this would be in my opinion a low-cost way solve a bunch of issues. In my case it would allow generating landscape tables with a landscape ref doc. If nice use cases emerge, then proper arguments may be discussed in the high level function gt_save later.