[utils,common] Merge format_srt_time and _subtitles_timecode

format_srt_time uses a comma as the delimiter between seconds and
milliseconds while _subtitles_timecode uses a dot. All .srt examples I
found on the Internet uses a comma, so I use a comma in the merged
version. See http://matroska.org/technical/specs/subtitles/srt.html and
http://devel.aegisub.org/wiki/SubtitleFormats/SRT