[ext. subs] Check for delimiters (Ticket #15011)

This fixes trac ticket http://trac.xbmc.org/ticket/15011
Only https://github.com/ace20022/xbmc/commit/d4d0f4b4781adfdc392f3416a80e22aff61b2509 is relevant the other are skipped after https://github.com/xbmc/xbmc/pull/5204 got merged.
@Voyager1 @Karlson2k for review
If I'm reading this right you don't load any subtitles that doesn't start with filename + one of those delimiters?