Adding support for frame-ancestors

[JavanXD]

First of all - thanks April! It is a very helpful add-on, exactly what I've searched for. I was able to create the policy within only 5 minutes.

The use of the X-Frame-Options header and Content Security Policy’s frame-ancestors directive are a simple and easy way to protect your site against clickjacking attacks.
Reference: Mozilla Web Security Guidelines (X-Frame-Options)

The X-Frame-Options header will be soon obsolete as it is replaced by the frame-ancestors directive.
Can you please add support for this frame-ancestors? The observatory service is already validating if a frame-ancestors directive does exists.