gst-play: load local subtitles

Submitted by Víctor Manuel Jáquez Leal `@vjaquez`

I don't want to bloat gst-play, but, at same time, I think this little snippet
might be useful as implementation reference.

Basically it mimics mplayer's functionality, loading the subtitle file if it
is present in the same path of the media file with 'srt' extension.