support setting values in dBa

[n1trux]

it'd be nice if you could set the volume not only in %, but also in dBa. pavucontrol displays values in dBa, too, and pulseaudio supports it. One probably has to write a wrapper for Alsa, though.

[hastinbe]

I did consider supporting dB volume in the past, but the demand for it hasn't justified the work to do it (this is the first request for it). I decided that supporting a single volume type was easier (differences between PA and alsa-utils) and that % volume was the most user-friendly for the average user.

With that said, I'm still open to it and welcome any PRs, as I want support for as many use cases as possible. It's a simple idea that requires quite a bit of work to implement.