Add $permissions modifier

[ameshkov]

It would be really useful for implementing stealth mode features.

More info:
https://caniuse.com/permissions-policy
https://www.w3.org/TR/permissions-policy-1/
https://developers.google.com/web/updates/2018/06/feature-policy
https://w3c.github.io/webappsec-permissions-policy/

TODO: we need to come up with a better description of what we want. It could be used as a part of advanced tracking protection indeed, but we could also introduce a new filters modifier (just like $csp)

• Once this feature is done, we should create a separate issue in the tsurlfilter repo
• We should add a test for it to testcases.adguard.com
• KnowledgeBase should be updated

[sfionov]

Link from #174:
http://xhr.featurepolicy.rocks/