Add style argument to include_url()

[jvcasillas]

I would like to suggest including a style argument to the function include_url(). By default iframes include a rather ugly border around the object that is embedded. I propose including a style argument so that something like style="border:none;" can be included. If this is something that would be desirable, I would be more than happy to submit a pull request, though I don't know where include_url lives.

[yihui]

This sounds like a reasonable feature request to me, but I don't have time for it. If you want to disable iframe borders globally, you could just define it in CSS like (I think you certainly know it):

iframe {
  border: none;
}

[yihui]

Done. Example:

```{r, out.extra='style="border: none;"', out.width='100%'}
knitr::include_url('https://yihui.name')
```

I think you still want to do it in CSS because that's cleaner.

[github-actions]

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