[New sniff] Check for the use of iframes

[carolinan]

Rule:

WARNING : Using iframes is discouraged. All resources should be included in the theme. iframes are sometimes used to load unwanted adverts and code on your site.

Ref: https://make.wordpress.org/themes/handbook/review/required/#stylesheets-and-scripts

Theme check file covering this rule:

https://github.com/Otto42/theme-check/blob/master/checks/iframes.php

To do:

• Create unit tests
• Create new sniff