Add volume control

[klinki]

Hello,

I like your library and I needed to add volume control. I thought it might be useful for you, so here is PR.

[rxlabz]

thanks for the contribution, and sorry for the delay ! I just want to look if I can add something similar to the iOS side before merging it. I'll try to do it this week-end.

[eplnh]

so, when will you provide volume control ???