Merge 2 srt into an ass

It would be really nice. Something like [this script does](http://www.abclinuxu.cz/blog/Joseho_blog/2007/7/30/188299). Here it is the [use case](http://yuenhoe.com/blog/2013/04/playing-multiple-subtitles-simultaneously-with-mplayer/), using toptitles and subtitles to play at once two subtitles in a movie file.

Thanks for your gem!
