Store BOM in extracted srt subtitles

[Petr Nejedly]

When extracting srt files from mkv via ffmpeg, ffmpeg doesn't include the BOM mark in the output file. It then breaks on some devices. We should make it default, or at least settable in profiles.

http://forum.serviio.org/viewtopic.php?f=5&t=19300#p102284

[Petr Nejedly]

This is done for subs that get converted to different encodings. Should do it to utf-8 to utf-8 files too.