Implemented query search for opensubtitles

Opensubtitles was only using the hash search functionality. For series,
it will now search on series name, season and episode.
Make hearing_impaired option more flexible

Hearing impaired has now 3 values:
- True -> download only hearing_impaired subtitles
- False -> download no hearing_impaired subtitles
- None -> download all (do not check hearing_impaired value)

-> None becomes default value in download_best_subtitles

-> Fixes https://github.com/Diaoul/subliminal/issues/343