play-1.js: add support for speedup ratio

[joaocc]

This one is quite simple to explain.
We would like to be able to speed up / slow down the playback by a factor of N. This could be made available to users via a slider.

After reading through play-1.js, it seems that the only way to do it now is to redo the abc conversion with a different BPM setting.
Something like "set_speed_factor" with a float (<1 == slowdown, >1 == speedup) would be great.

Thanks

[moinejf]

Your commit should be in the issue #21.

[joaocc]

You are correct. Sorry for the mixup. Will remove the comment to prevent misleading link.

[joaocc]

Not sure if this is the most elegant way to implement this, but we tried to keep changes to a minimum.

[moinejf]

I already implemented such a function in a previous player and I was about to code it.
(your proof of concept does not work because the realtime origin must be shifted)