[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 )
same needs to be done for .idx subtitles few lines down

```
CStdString strSubNameIdx = StringUtils::Format("%s.%s.idx", strFileName.c_str(), strSubLang.c_str());
```