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.
Merge branch 'embed-sources-675-676-678' of github:Darael/dw-free into embed-sources-675-676-678
Add test to finish fixing #678

Test URI is correctly-formed but not a real link (none was provided)