Bug for presentation mode

You've written a great plugin that solves a real need. Thank you!

I tried to load my slides in presentation mode by swapping out `/edit` with `/present` as instructed in https://productforums.google.com/forum/#!topic/docs/wSI6zxGWz5k

I tested that the links worked in my browser `as-is` before putting them in gitbook.

The url with `/edit` shows up fine but when the URL has `/present`, it does not load and I see the following error in the console logs for chrome browser:

```
Refused to display 'https://docs.google.com/presentation/u/1/d/11gkl1nfHHU1UQnd1n6Z4IaWeKA8PniqMf3TRTbgwe6c/present?rm=undefined&slide=id.p4'
in a frame because it set 'X-Frame-Options' to 'SAMEORIGIN'.
```

Feel free to use my real presentation URL for testing it yourself ... it doesn't have anything super secret in it. cc @harshadyeola