Fix #675: add archive.org as an embed source

Includes one passing test but could probably use the addition of a few more?
Fix #676: Add video.yandex.ru as embed source

Not speaking Russian I can't read relevant documentation to make sure
I've got the formats of individual video URI-components correct but I've
gone for an approximation based on what examples I can find.
Fix #678 (add episodecalendar.com as embed source)

Test URI will 404 but is correctly formed

Test URI is correctly-formed but not a real link (none was provided)
Merge pull request #718 from Darael/embed-sources-675-676-678

Add embed sources (fixes #675 #676 #678)