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)
Updated name variables and cleaned up a bit

Updated name variables and cleaned up the code that fixes
subtitles_subliminal plugin
Merge pull request #386 from camon/develop

Fix language code prob and add extra config options to subliminal plugin