Implement <amp-ad type="adform"> support

[simast]

This PR adds Adform ad support to <amp-ad> tag.

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[cramforce]

Would you mind instead changing validateSrcPrefix to take an array of prefixes? And add some unit tests there?

[cramforce]

This looks excellent! Thanks! I added a few comments.

[cramforce]

Please rebase. I changed the amp-ad docs around a bit.

[simast]

@cramforce: Squashed and updated per your comments!

[cramforce]

Excellent! LGTM