add volume control to pipeline

[itsb]

Addresses #2.

[gmsoft-tuxicoman]

Thanks for your patch. Wouldn't it be best to change the volume porperty of the sink instead of changing the volume of the audio ?

Moreover, there are scenario where you don't want volume control if it's handled separately. For example I use a script that uses the volume properties and sends the right serial commands to my sound system. Please add an option in the arguments to disable it.

[itsb]

The alsa sink currently in use does not seem to expose volume control, see docs. That said, I was able to use sink volume control with the pulse audio sink added in this fork, see pulsesink docs. Please let me know how you want to proceed. Should we merge the pulseaudio toggle first and then add the volume toggle?

[gmsoft-tuxicoman]

Sure pulse audio toggle works for me. We can commit that first. However pulse audio already has support for BT and volume support, make sure they don't interfere.

[itsb]

cherry-picked Hubert's --pulse toggle, and added my --volcon toggle on top

[gmsoft-tuxicoman]

Thanks !