Play music/mp3 when starting metronome so its synced

I think it would be very helpful to have the song play along with you. Now I don't mean have the watch detect the tempo of the song, but even something as simple as adding your own song and lining it up so that you can atleast play with it. Turning on the song to the start of the tempo is really hard to do and I feel this would help many people.

Changed the title from "Play music when starting tempo" to "App: Play music/mp3 when starting metronome so its synced"

Merged "Music with the beat." #529