Allow music to be played in background for Safari on iOS

Hello and thank you for this amazing project !

To date, when playing audio from Safari on iOS, webamp music can't be played in the background. Not even with this [*"manually playing with the control center"* trick](https://www.youtube.com/watch?v=G0ZRDIe_xoU).

I am not talking about autoplay, but only playing in the background after pressing the "play" button.

A lot of web-based players can play in the background, for instance the native player on archive.org.
I am clueless about HTML/JS but it seems that Safari on iOS needs a proper `` tag in the HTML of the web page.

Can you adapt webamp to allow playing in background ?

Cheers!