news: looser relevancy for entities.

cc @bsstoner @russellholt @nilnilnil @jagtalon 

allow looser news relevancy when we have a good signal that the query contains a newsworthy entity.
`DDG.news_entities` will be set internally.

this is pretty far out of my domain, so if there's a better way to override (not in the technical sense, but in the sense contained herein) `DDG.isRelevant` (or if my JS sucks) please let me know. moreover, not sure who to assign.
Nevermind, IE8 requires a polyfill :frowning: