iframe in - rewrite results - Override the output of this field with custom text
I have been wrestling with this as well but my users want to embed google docs.  Same concept though.

I figured out the solution and it's not super complicated.  You you just need to create a 'Field Template' in the theme. 

This video covers the concept with 'Node Template' but you can easily apply it to the field template.

Drupal 8 Theming - Part 11 - Displaying Fields
https://www.youtube.com/watch?v=fQ6Ypik_jsE