Fix language code prob and add extra config options

There was a problem with Language codes, e.g. pt-BR was not supported.
Subliminal already supports this by using IETF codes

Added two extra options (that are not mandatory off course):

providers - allows to chose specific providers that are used for
subliminal to download subtitles

single - allows to download subtitles in single mode (no language code
added to subtitle filename)