[coreSubs] add option to 'Add language to filename'

- This options can be found in subtitle addon for frodo. ( http://dl.dropbox.com/u/19359762/XBMCSubtitles/s3.jpg )
- It was asked for the newer versions too.( http://forum.xbmc.org/showthread.php?tid=197440 )
Yes, you are right. I missed it, sorry.